Those 2x4 ceiling joists just look scary... My garage has 2x12s for the ceiling joists... I suspect that the reason that they can get away with the 2x4s is the truss system that you have... Still, these types of trusses are for loads in compression, not tension...
Personally, I would not use a 2x4 across the ceiling joists like that. Maybe a 8 ft 4x4 would be acceptable, especially if you centered the eye bolt between two of the ceiling joists. Also, don't use the cheap eye bolts that are just bent closed. Go for the forged ones or even ones that have been welded closed. One of the problems with a 2x4 like that is that if there is a knot in the board, it is not going to take much of a load to cause the board to break. Even better than a 4x4 would be piece of metal pipe or heavy wall square tubing. I was lifting up the front end of my sportbike the other day to change the front tire. Since it is a fairly light bike (about 400 lbs) and since I'm only picking up less than half of the weight, I just went with a ratchet strap looped over the 2x12 ceiling joist. I am now in the process of changing the rear tire on my Harley FLHP-I. It's an 800 lb bike, so I don't trust the ratchet straps even though they are supposedly rated at 3000 lbs each. Plus, I need to raise the bike at least 13" off the ground in order to have room to slide the tire out the back (the rear fender is only 12" off the ground and the tire is about 25" in diameter). Doing that with a ratchet strap is not going to work that well since the spool is probably not large enough to take up that much strap. I would probably need to use the strap to winch it up a bit, put some blocks on the bike, loosen the straps and re-cinch them, and then winch the bike up a few more inches, repeating this until I got it to the desired height. Not necessarily a quick endeavor. Since I'm going to be basically underneath he bike working on it, I want something that is going to make me feel a bit more comfortable than a couple of nylon straps. There's also the issue of my not seeing anything all that readily visible that I would want to anchor the straps to on the bike that I would trust to hold the weight of the bike while also not damaging other parts of the bike when the straps were pressed against it under load.
So, my initial solution was to take a floor jack and position it with a short piece of 2x4 across the bottom of the bike so that the weight was being supported by the frame rails of the bike and then centering the 2x4 on the pad of the floor jack. Now the real problem with this solution I soon found out is that it's quite difficult to get a good balance point that you can trust while sitting on the ground with an 800 lb bike wanting to either fall over on you or fall away from you while you are operating the floor jack with one hand and holding onto the bike to try to balance it with the other hand. And to make it even more interesting, since the floor jack has wheels, the bike moves a bit sometimes when it is leaning. I think my bike moved about 30 degrees before I finished my various attempts at this. This method basically had me jacking up the bike a few inches and then stacking some 2x6s under the bike forward of the jacking point. This might have worked a bit better if I had another person to balance the bike or to handle the sliding of the wood pieces under the bike. I even tried using jack stands at one point with them supporting the rear footboards, but they could not extend long enough for the height that was needed and I didn't want to put blocks underneath them to raise their height.
Eventually, I determined that instead of using a 2x4 underneath to jack upon, I should use a 4 ft piece of 2x8 that I had instead. I would jack it up, put a couple of wooden blocks under both ends, and then repeat this until I could replace the wooden blocks with cinder blocks. Once I got it high enough for the cinder blocks on each end, I changed from using the floor jack to a bottle jack on each end of the 2x8. The bottle jacks made it a LOT easier since I didn't have to worry about extreme imbalances anymore. With 3 ft between the jacking points of the bottle jacks, even an inch or so difference in the heights did not create an angle that would make me be concerned that the bike was going to fall over. As I would jack up the 2x8, there would be a space between the 2x8 and the cinder block and once it was enough to put a 2x4 block in there, I would add one on each side. It took two 2x4 blocks on each side and a 1x4 block to get it to a height that I believed that would be sufficient for removing the rear tire straight out the back under the fender.
One thing that I did notice though is that it would be a lot safer if you had a wheel chock bracket to hold the front tire in place. Without any sort of wheel chock, the bike moves around a bit too much. A brick works better than nothing, but I believe one of the wheel chock brackets like you see for trailers would work best. Having it attached to the floor and strapping the front tire to it might have resulted in a bit less adrenaline on my part during certain parts of this endeavor. Having another set of hands would also be a great help.
I think that a better solution for the 2x8 lift bar would be to have one that had some "wings" on the side that would allow the lift bar to be initially lifted with the bottle jacks and not with the floor jack. Here's a quick sketch of it... The green pieces are 2x2 square tubing, 1/4" wall thickness. The yellow triangular pieces are corner supports and made from 1/4" thick steel. The red pieces are the actual jacking points for the hydraulic bottle jacks and are made from 1/2" thick steel. They are also 6" long by 2" wide. Depending upon how good of a welder you might be, the corner braces might not be necessary. I'm a fairly crude welder (i.e. I can make it stick, but it's not necessarily that pretty), so I add the additional support.
Of course, you could probably get away with lighter materials, but I figure that anything worth engineering is worth OVER-engineering. Plus, it's my legs that are going to be broken if the thing fails and the bike falls on me.
By adding a ring on the top of the two jacking points, you could also use this bar to lift a bike from a ceiling mounted winch or whatever. The bar would keep the straps / chains / cables far enough apart that you would not have to be concerned about them damaging your bike in the process. Maybe something like this:
With this, I changed the triangular support pieces to 1/2" square solid bar stock. There are two of them on each side of the lift bar and they are welded to the surface of the bar. The purple colored rings are made from 1/2" thick rod (or even rebar). They could also be made from a chain link that was cut from a chain and welded to the top piece. They are shown this way, but they would be even stronger if welded so that they overlap the vertical pieces while also being rotated 90 degrees so that the loop is perpendicular to the horizontal axis of the lift bar.
On the other hand, if you have 2 floor jacks, you could probably get away with using just a 2" bar (or 2x8) across the top of them and it would still fit under the bike and give you room to jack. Even though the 2x8 seemed strong enough for my endeavor, I would feel more comfortable with making it out of steel. When wood fails due to a knot, it just snaps and everything falls. When steel fails, it starts bending (unless it is a weld failure).
