[69heavychevy]

I went through this same issue when swapping over to hydroboost. I wanted to use an aluminum MC with the plastic reservoir. Mostly for the ease of keeping an eye on brake fluid level and avoiding the rusty look of the original cast iron MC.

The piston bore on the various aluminum MC were all too large to mount into the HB unit. So I just ended up going with the cast iron MC unit it called for.

The GMT400 HB units mount up perfectly but the issue is the pedal rod is different. Those units do use an aluminum MC, they're essentially identical with minor external differences. I'm sure a newer HB unit can be made to work fine on these trucks, but I just didn't feel comfortable experimenting with my brakes.