From my experience with boats and their alternators, I don't think you need more than a 100 amp alternator. You might look at a West Marine catalog to see what amperage they advise using multiple v-belts; cruising boats often have big alternators and require multiple belts to drive them. I think it is around 100 amps. I think the loads cited above may be a bit high for some of them. The largest draw on a GT40 seems to be the multiple front lamps, if you don't have AC. Even with AC, the biggest load is on the blower, which isn't that much. Mind you, it will run all the time, since between the large windscreen and the engine right behind the driver, the heat load to the passenger space isn't small.
One way to do this would be to find out what ampere spec alternator SPF uses. For marine use, I have found that 65 amps seems to be plenty, and that runs multiple lights, the navigation systems, various pumps, toilet and treatment system, VHF radios, etc. Of course all that stuff is not working at the same time, and the battery bank depth is considerably more than in a GT40. But very few GT40s have their own toilet and treatment system....