what size mega fuse?

if 100a is all the alternator can produce then you can use a 100a maxi. in a street rod i doubt you will need more than 100a. if for some reason you DO have some componet that will draw more than 100a, like a high powered amp or amps, it will burn the fuse. if you were to put in lets say....a 150a maxi on your 100a alt, the fuse will hold and it will just continue to tax the alternator over what it can produce eventually prematurely causing it to fail. the purpose of the fuse really, is to protect the alternator and the wire from it against a direct short to the chassis. so, that being said i would always run a larger fuse rating than the alt rating. after all...even with say a 200a maxi, a direct short will pop it (which is what its for). as a rule of thumb, for what you are doing with it, go bigger as it really is only to protect against a direct short anyway.