From the sound of it, he has a complete setup. Rare, bare manifolds go for $500+ on eBay. This is no manifold, and is probably one of the few survivors. Don't mean to spoil your fun, but I would think that a complete setup for a rare piece like that has got to be worth atleast $5K. But then again, since he is a friend of yours, maybe you could get some labor/parts trade in addition to some cold, hard, cash. This is a rare piece, and if you don't go through with putting it on your rod, I would suggest acquiring it as an investment. These dont come along everyday.
Ilya