My '74 2.0's PO put in 320i front calipers. I went to replace the pads and found that one of the calipers was frozen with the piston out (pic 1 below) and I cannot get it to move. Good news, he also gave me another caliper (pic 2). Bad news, the caliper has a spacer (for vented rotor?) and the flanges are not machined down. Good news, I can take the spacer out, bad news, don't I need an o-ring where the two halves meet? If so, where can I get them? I'm hoping with those that I can then put the halves together, get the flanges machined, and I'm back in business?
https://1drv.ms/f/s!AuJyTaQROWZRg8R810bU27sxlPvkSg

You probably want to contact our brake guru Eric Shea at PMB Performance.

The piston can be pumped out using a grease ,thread a flexible (rubber)hose on to the grease gun for this procedure ,use a small G clamp to hold the opposite one from popping out.
Take care doing this as I once cracked a caliper half when the piston didn't come out straight .
Yes you do need small 'o'rings between the caliper halves,a brake repair shop should have the rebuild kits or BMW should stock them.

Why not put the -4 calipers back on? You have a set up that was an urban legend. Go to the classic section and read up on brake systems.

Check your local O'Reilly's or similar.....something like $35 for reman'd 320i calipers....

If you do this you will need to machine the mounting face. I can't remember but it is something like 1/8 inch to center caliper over the rotor.

Interesting ,read that on here in the past is it because larger pad area or bigger pistons? or is that not correct and it isn't an upgrade in reality????

Yes, it was not really an upgrade. Those crazy engineers usually went for the best option when braking.