What size are the wheel cylinders on the 14-bolt compared to those on the Eaton? If they are the same size then a dual master with both sides having the same bore as the original would be the easy answer.

For my '54 1-ton I'm using a 14-bolt in the rear and a C-20 IFS with disks in the front. Essentially my brakes front and rear will match factory configurations available in the mid-70's. My plan is to buy a dual master for a truck that had the same brakes front and rear as what I have.

I'm going to trust that the GM engineers knew what they were doing. While we all know from seeing/reading news of recalls that factory engineers are not infallible, I think we can follow their lead in sizing things like brakes and expect to come very close to the right combination of parts.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...