Looks like the same Master Cylinder as I have on my 1960 P-20.
I think I just got my Kit from the local "NAPA" Parts house.
But I noticed what you are saying also - most Chevrolet Trucks of that time period has a different Casting and Mounting for the Master Cylinder than like what you have ( and I have ).
Get yourself a heftier return Spring while you are at it, too.
Your Pedal return is a mite languid...and should be a little more definite and strong.
I Flushed and Bled my whole Brake System the other day using a couple feet worth of clear Plastic Tubing, running it into a Bottle.
Tubing slips over the end of the Bleed Fitting...crack the Bleed fitting a tiny bit, get in and Pump the Brakes, MC 'open' so you can watch, re-filling it as need be...then, once you figure the line to THAT Wheel has had enough new Fluid to run all the way through, close the Bleed Fitting and go on to the next Wheel.
