The FMJ changeover is about the same as A or B bodies, but the parts are a bit harder to find (sense most FMJ's came with power brakes).
Because the brakes are power assisted, they have a step up linkage (not sure of exact name) that causes brake motion to be doubled.
a crude example.
The brake petal is the same but master cylinder uses the lower bolt mounting holes, and therefore sits lower on the firewall.
The plate and pushrod are the hard parts to find (if converting to manual brakes).
If you reuse the “Z” style linkage – then you will find it very hard to push brake petal down.
I would look for an adjustable style pushrod if you still can’t find one to fit.
I’ve converted a few of these cars (not FMJ) over from manual to power brakes and should have kept the old parts (plates and pushrods) – but didn’t.
Reminded something about hindsight, not being what it used to be . . .
BudW