Power Brakes Conversion Help

[wareagle_1997]

I am restoring a 70 SWB and wanted to convert my manual brakes to power brakes. There are four mounting studs coming out of the firewall that are 2 1/2 inches apart from center to center. My manual master cylinder is only using the bottom two studs, and there is a hole in the firewall centered exactly between these bottom two studs. The problem that I have is that every power booster that I've come across from Napa to Autozone to LMC has studs coming out of the power booster that are 3 1/2 inches, center to center, and the pushrod is directly centered in the four bolt pattern, not centered with the bottom two bolts. But I have every reason to believe that I have a completely stock setup. Can anyone tell me what I'm missing here?