power booster/master cylinder help

[tropa]

just got done doing my drum to disc conversion on my 64 c20, now down to brake lines, power booster and master cylinder. I got this booster at the junk yard for $16 bucks at the junk yard off a 1967 c20 that had drum all around. went to auto zone and bought a new master cylinder and they said that this is the one i need if I have disc brakes but the problem is the push rod (if that is what it is called) is too long even if adjusted, looking online, this master cylinder also fits 1973 and above. the original master cylinder was smaller and was deeper where it connected to the rod. would it matter if i buy the original master cylinder with the new disc brakes and return this one in the picture?

thanks in advance.