what amperage alt should i run?

The ammeter bypass doesn't matter to the load, it just has you "flying blind". Whatever you do, buy a cheap cigarette lighter voltmeter to monitor as you drive.

A radiator fan is a big load, but only when it is running. That depends on operation, climate, ... On the highway w/ a good radiator, it shouldn't run. The alternator doesn't have to match the peak load, only keep the battery charged.