I've been driving my '37 for 26 years with a stock amp gauge, alternator, and recently installed A/C. With a good battery and proper sized wires, the amp gauge should stay right in the center, it will go to charge after start up while the alternator recharges the battery, but it will fall back as the battery recovers. Any accessory should not alter the amp gauge once the alternator catches up to the draw on the battery. My truck has HEI ignition, all the lights of a normal truck, a few add on gauges, A/C compressor clutch, and A/C blower motor. I have a small fork truck style alternator that puts out 40 amp max, I've never seen the amp gauge above about 10 or so and that's only after it has sat for weeks not started. The amp gauge is in its own metal housing, so I have no idea if it gets hot or not. The A/C pulls power at start up, but once running the amp load is pretty low, same for the HEI, head lights are probably the biggest users of power.
We would need to see the wire diagram to figure out how to bypass the amp gauge for the electric fan. The alternator should see any draw on the battery, so even relays won't help. Pulling power off the alternator before the amp gauge might work.
If you are worried about the heat, just put it together and try it while checking the gauge with a heat gun
