Brake experts wheel cylinder size?

[T.B.A.]

Hi all, I've been lurking around here for a while and have learned a lot in the process. What a great site!

I've got a 69 CST 10 SWB with a 454 th350 and 12 bolt combo that's just got back on the road. I bought it less engine, exhaust etc. (was originally 396)

One of the p/o's at some stage installed 6 lug front disc brakes.

The diff currently installed is a 4:57 and not sounding healthy.
It has, I assume the wrong drums (no adjusting windows) and there's no windows in the backing plates either to get the drums off to check what cylinders are in there.

I've sourced another 12 bolt with a 4:11 posi' centre that's getting overhauled now, but I'm trying to have this complete, fresh, drum to drum so I can just install.

Anyone have any idea what the rear cylinder size would be?

The front discs are the over an inch thick type, large single piston calipers with a corvette style master and NO bias valve.
It seemed to stop ok, never locked a rear wheel up etc.
Has anyone else done a similar set up?