HAH! Sorry for chiming in here, but...
I
know what you mean. I just attempted (front) wheel cylinder replacement on my '64 C20. I got the cylinders 4hrs after ordering them, and took 'em home to replace them. I strip the front wheels to the backing plates, and remove one of the wheel cylinders, and replace it, only to find that the long barrel fitting on the (newly replaced) brake line does NOT permit reaching the (
metric) bleed nipple on the new wheel cylinder(s). It's a 9mil, and
short. As a result of them being metric, you can NOT use the original (long hex) bleed nipples. BAH!
If you still have your original wheel cylinder(s). Keep them! I have the number for a couple of manufacturers (BENDIX being one) that still make the
correct replacements.
In the end, I just opted to rebuild them. The cylinders weren't so bad that a good cleanup, and rebuild wouldn't cure (and did).

Why did America create SAE, if they weren't going to continue use it?!
Chris
ACDelco wheel cylinder rebuild part number (all 4 wheels) ACD# 18G5