OX CNC with 1/2" aluminum plates

I don't have time to go through all the screws right now but can offer information such that you can calculate them yourself.

wheels - 11.0mm
eccentrics/short spacers - 6mm (or 6.35mm if you go with 1/4")
long spacers (middle spacers on X-carriage) - 9mm
nuts - 5mm

So for the long bolts on the X-axis carriage = 12.7+6+11+9+11+6+12.7+5 = 73.4mm which may be a problem because the flat head OpenBuilds screws don't come that long. And while socket head screws are available in sufficient length, you wind up with having to deal with the screw head protruding a substantial distance from the face of the plate which can cause issues. The only other option is milling recesses for the screw heads and nuts. For the screws at the fixed wheels this is not a big deal. (I use the Dewalt pilot point bits as the larger sizes will drill a nice recess hole with relatively square shoulders.) For the screws at the eccentric adjustable wheels, it won't be quite so simple. You'll need to take your router and mill a proper (and flat) oversized recess. But you only need to do 3 of them so it's not necessarily a deal breaker. I would suggest using the 70mm screws and only recessing the head end.

As for the gantry end plates you should be fine just using longer screws. The only issue I see with the pulleys may be having to tighten the pulley set screws prior to attaching the stepper to the plate.

 

Sorry, you are correct with there only being two bolts top and bottom on the X-carriage. (I've gotten too used to seeing the C-beam carriages which have 3 rows and haven't actually thought through an Ox in a while.) And looking at the diagram you should be able to just go with 80mm socket screws as protruding heads won't be an issue. There is no need for any recesses.