383 = no love

I just had a 383 rebuilt and needed an intermediate shaft.
I bought what is supposed to be a Melling IS63 intermediate shaft.
Is there some secret to getting the correct shaft?
Are B motors and RB motor pumps different?
I can look at it while the intake is off. Do I need to run a depth gauge down in the hole to see were the bottom of the intermediate shaft would stop in the pump or something?