Isn't the stud spacing on the Astro HB 3.40"? While the C3 Corvette master cylinder has the same 3.40" hole spacing as the hydroboost (and 2nd-gen Camaro/Firebird), the former is a disc/disc unit and the later is a disc/drum unit, but either would probably work for your application. The Ford MC's noted above by phred have a 3.20" or 3.25" hole spacing if I'm not mistaken, and would require those holes to be slotted before they would mount up, so they wouldn't be my first choices. Bore size is the next question, as the early GM stuff (C3, GMF2, etc) will most likely be Ø1.125" whereas the 1990-1995 Astro units are Ø1.250", with an approximately 23% larger effective piston area (lower output pressure, shorter pedal travel).
What disc brake calipers and drum wheel cylinders are you running? The size of the slave cylinders in conjunction with the other brake system variables (booster, pedal ratio, etc) will set the hydraulic requirements of the system with respect to the master cylinder bore size, but if you like how the Astro unit works and feels, then staying as close to that as possible would be a good.
I don't personally love the snap-loc lid design of those GM master cylinders, nor do I prefer the stepped reservoirs found on the Astro minivans for hood clearance, but if you're looking for a direct replacement with a better (IMO) reservoir configuration, I think there were some similarly sized units for the 1988-1998 C1500 applications that could be a good option.
