FWIW, I'm of the school that says the center bearing is the source of most Mangusta drive bearing problems. If you're worried about the shaft whipping from the combined alt/AC load (that from an alt alone on a shaft that size is almost non-existent), use a home-made nylon support bushing in the center position. I suspect that heat-related expansion/contraction of the engine castings causes more distortion in the long, spindly bearing holder than the center bearing can compensate.
Good to see you're working on the old 'Goose, Terry. Let us know when it makes it around the block on its own power and I'll hoist a drink for you!