I didn't think about angle iron, but that could be a good idea. I just thought of another way to strengthen the 2x8. I can buy two 72"x2" readily available Everbilt 1/8" thick steel plate at Home Depot and run my circular saw along the middle of the sides of the 2x8 (2 inches deep) and then insert the plates on both sides. If this works well, I'd want to experiment using 2x8 redwood which is a lot lighter than fir.
I tried reinforcing one of the 2x8s with 1/8" thick 1.25" x 72" long steel flat bars. I placed one bar on each side of the 2x8 and screwed them down 6" apart along the whole bar on each side (using 2.5" long screws). No difference; the deflection appears to be the same, if not a smidge better. Then I tried placing an additional block on the center between jack points. This seems a lot more solid because the center gives the extra support. But the area between the center and the front end still show very small amount of deflection.
For the Odyssey, anywhere between the front and rear jack points is a strong pinch weld so this additional block solution can work, but I'm sure it won't work for all cars. However, I'm still skeptical about the slight deflection of the wood and will always feel a little weary about it so I'll proceed with ordering some steel and getting someone to weld it. Once I get a successful adapter, I will post the exact specs that works.
Based on what I can see, I'm pretty convinced a 2x8 can work fine for lighter cars maybe 3,500 lbs or less. But then again, if the car is that light, chances are it fits the QuickJack BL-5000SLX's 60" span without the need for an adapter (or just needs an extra 2-3 inches at most). For those who are just inches short with a car of average weight, you can easily trust a 2x8 (cut to your desired size).
Eddie777, I suppose you haven't tried a car as heavy as the Odyssey (around 4,500 lbs) have you?