I think the biggest risk in using a drum MC on a disc car, is that their smaller sized primary reservoir might run low on fluid as the pads wear.
On a car that’s not driven a ton and whose owner checks and replenishes fluids often, that’s not likely to happen.
The B body disc MCs of that era were 1-1/8” bore and the drum MCs 1”, you may find the pedal feel and grabiness of the brakes not to your liking, but without trying it, it’s hard to say for sure.