Yes I mean the hole in the piston where the booster pin goes. I dunno if that is deep or shallow. I never measured. Sounds deep though... or in between.
This Camaro site has some good pics of the differences on the 2 masters and GM booster pushrods:
I think the 'Vette masters are both deep hole, though I'm not positive on the P/B master. That could be one reason a lot of companies use them. They also have decent volume distribution and cylinder stroke.
I think Baer makes pushrod extenders if yours wont extend that far. Did you figure out how to get the pedal pushrod to work on your truck with that booster?
Also, how much line pressure you need really depends on what brakes you have at the wheels. The overall ratio (cylinder ratio x pedal ratio) and the booster are really what's important.
With that pedal ratio, 2 15/16 single piston calipers, and the tiny Fox body booster, you may be just right with the 1.125" master. It really depends mostly on the size of the caliper pistons and the effectiveness of the booster. You may be happier with a single diaphragm booster at that pedal ratio, or maybe a larger master.... I still don't know what front brakes you have. I'm just assuming the common 70's GM single piston.
Of course, even the big car companies have bulletins and recalls to change master cylinder sizes, booster diameters, friction compounds, or balance valving occasionally. No matter what you choose, it is an experiment the first time, but math and common sense can get you in the ballpark, and perhaps perfect the first time.
Dont put too much faith in the Q and A section of MP, though it is good as a general guide for most (but not all) GM midsize, fullsize, and 1/2 ton, a lot of their A's are inaccurate or dead wrong for many Ford and MOPAR, as well as some GM.
(I'll cite one example. 1973 Dart master cyls. 1 1/32" manual disc, 15/16" power.) There are others, and other A's I could crush, with specific examples and reasons for them, but it is okay as a guide for most people. Mostly all they really need to do is add 'usually', or 'often' to more of their answers. I guess everyone knows there are always exceptions. Sometimes I just go off. Sorry. :end rant: