Master cylinder ID?

[chevy72blu]

Getting the truck back on the road after sitting for 6 years, its probably a good idea to go through my brakes. I'm noticing a bit of sponginess and "pedal sink" while sitting at stop lights, so I've narrowed it down to the MC and fluid flush (I put new pads and shoes on with only a few thousand miles on them before it sat, so I see no reason to replace them unless there is a problem).

I am 99% sure I have a 1 ton booster, but I am not sure what MC to buy as a replacement. Any ideas? I'm thinking 73-87 3/4 ton