Good Morning to some and Good Afternoon to others.
this pulley thing is simple...stock gen/alternator is 4 1/4" OD, but found some early pulleys measured 4 3/16" OD
CB performance billet pulley measures 4 1/4" OD
356 porsche is 3 3/4" OD.
stock steel crank pulley [with grooves and no seal] measures 6 7/8" OD
JC measures 7" OD with or without grooves for a seal.
Scat alum pulley measures 6 3/4" OD either with grooves or without for seal.
aftermarket "power pulleys" measure 5 1/4" and were designed to clear dry sump pumps and slow the fan speed and resistance at higher rpm's.
The popular trick of the days past was to use the 356 pulley to speed up the fan speed at lower rpm's. of course it also increased fan speed overall but the negative point was the fans would come apart.
Overall I have ran all these combo's and they were suited for each situation. there's not one correct application unless it's 100% stock. Same logic goes for the seal no seal topic. what ever works for your application. several people feel they need a seal at the front pulley to eliminate oil leaks. Most cases your oil leaks are due to TOO Much Crankcase pressure.
Hope everyone is doing well with all the things going on. no better time to work on that classic.......just think of it as a job and when you walk out to garage for 8 hours you won't hear your other half ask " what are you doing" every 10 minutes