Hrmm I hadn't thought of the ABS hole. Suppose if you don't have it you just leave the spot empty.
So, if I'm understanding you correctly
-you've got the spindle mounted and in position
-rotor and hub in place
-caliper bolted on
And you're just having trouble get the pads in?
If your brake calipers look like the ones in this DIY guide
(
PeachPartsWiki: Front Brake Pad & Rotor Replacement) what I normally do is open up the brake reservoir in the master cylinder. Then grab a big flat head screwdriver, placing the tip of the screwdriver on the rim of the piston, apply pressure and push the piston back inside the caliper. Install a pad, and then repeat for the other pad.
On my 91 300D it has single piston calipers, so its just one big piston that I have to use a C-clamp to compress.
Sounds like a good plan, I'll throw him a link to this thread. Neither of us have worked on a W123 before. W201, W124 and W126 sure but not a 123.
Do you ever hit up any local junkyards for parts?
You should post your build progress on here more, post up pictures and whatnot!