[69 C10 LWB]

I spent a good portion of yesterday trying to bleed the brakes on the truck only to realize I am an idiot. Hopefully someone can tell me where the problem is but I managed to find a good solution. I used a vacuum bleeder to get fluid to all the wheel cylinders but I still had no pedal. The last 3/4 to 1 inch of travel at the pedal would start to do something but not much. I pulled the master back off and plugged it to bench bleed it a second time and it seemed fine. That is when I saw the problem. The pushrod only sticks through the firewall about 1/4 inch. The master I got has about a 1 inch counterbore in the piston. The pushrod wasnt actually touching the piston until the pedal was almost on the floor. There werent enough threads to just adjust the pushrod out so I ran up to the hardware store and bought a coupling nut, a fine thread bolt and a tap. Trying to find a fine thread coupling nut late on a saturday afternoon the weekend before christmas is like trying to find gold. I bought a 3/8 nut and drilled it out and retapped it with the correct 7/16-20 threads for the pushrod. Then I cut the head off a bolt and threaded that into one end. I installed my new pushrod and I now have brakes.

Now for the question. When I did this on my 64 I just installed the new master cylinder. No work other than adding another brake line to the rear.

Is there a difference between the round and the square single master cylinder?
Is there something wrong with the new master cylinder I got? Is the piston supposed to be flush with the end of the bore? 67 C30 master cylinder.