anybody know what size the bolt is for rear brake clearance in the trailing arm.

M6 allen if I remember right... I know what color it is in my color coded caddy!

Or a Craftsman chisel if you're not careful.

It is not a 6mm. I think it is a 5mm. The plug and inner adjusting screws take the same size allen.

If you still have the original plug, it is either 4 or 5 mm. I wouldn't bet the house on it, but I'm going with 4mm. They tend to strip and in the old days I would put a big
slot in the head for a screwdriver.

The adjuster inside is 4mm.

I just did this the other day.
And after looking at my brakes parts stash, there were three different types of caps before getting to the adjuster.
On the car was a cap with a 10mm bolt head cap with 4mm adjuster underneath.

One set of spare rears had a socket hex cap (maybe 6mm), while the third had a standard screwdriver slotted cap.

I just did this on my 70, 5 mm into the cap be careful not to strip it or else it's the chisel. adjusting screw inside is 5 mm also.

Yep, it's a 4 mm hex key to adjust inner clearance.