I understand what the OP is saying. Many (most?) 4 posts have the runways bolted down from the ends. There's mostly two types of 4 posts nowadays, "standard" and "wide". As cars and trucks have become bigger and wider over the years, any modern car (especially sports cars) now have an outer edge-to-edge tire width that exceeds the outside edges of the "standard 4-post" runways. I myself unfortunately have this problem, as the relatively-compact cars I had back in 2017 when I bought my lift didn't account for the vehicles I'd have today.
In the end, I haven't messed with it and just let the car tires sit just a little bit over. I normally only have one specific car on the lift, which fits mostly fine (tight, but fine), and the other cars are only up there when doing an oil change. So if the tires overhang a bit while just doing an oil change, I'm fine with it.
OP, if you look closely at your how your cables and pullies are setup, you may find that the location of the pullies may restrict you from being able to move your ramps, as they'd change the effective length of the cables, depending on which post they go to. So if you do decide to slide the ramps, take this into consideration. Also, if you do find a way to make it work, be weary of how many additional holes you're putting in your cross beams. You don't want to swiss-cheese it and end up with a weaker beam.
Also, in regards to the left/right angle iron to help guide your car in, be aware of your tire height, as bigger wheels/smaller tires means you may scratch the wheel if you rub up the angle.
Oddly enough, I've recently found myself parking my car backwards on my lift. I've found that while tricky at first, being able to steer while backing in has allowed me to do an excellent job of centering all four wheels on the lift. When pulling in forward, getting the rears centered was always tricky, but backing in is a much smoother straightening process.
