I have a '68 4sp 400 w/front 4 piston disc 2 1/16. I picked up a rebuild seal kit from Ames Performance (p/n FR218KB)that was the right size. I believe they have the 2 1/16 pistons as well.
I have run into an interesting situation with bleeding the front lines and it has to do with the master cylinder but I'll start a different thread.