OK- - - -I just looked up some specifications. 54-56 Cadillacs ran a 30 amp generator, which is probably the same as the GMC ran. Your regulator will most likely work without doing any swapping. Be sure to polarize the generator by momentarily touching a jumper wire from the BAT terminal on the regulator to the ARM terminal. (about 1 second is plenty) That's particularly important if the GMC is running a positive ground electrical system. The generator will work on either polarity as long as it's polarized after installation.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!