I put a hydroboost off a 2002 2500 Cummins, onto my 79' project with a 440. The two holes on the right fit in the factory booster bolt holes. The other 2 on the left I had to drill through the firewall. The brake pedal hooked right up. I did have to make a thin plate that fit between the hydroboost and the firewall. It pretty much covered up the 2 unused bolt holes from the original booster. You do need adapters in the hydroboost master cylinder, to go from metric to SAE.
I haven't got it all hooked up yet though. Still need to get hoses made, but I bought the hydroboost for $20, and $13 for the fitting adapters. I can't imagine the hoses being too bad for price.