There are others who know better than I about whether the 915 would make a good choice. There was quite a discussion a while back. If you would like some background of the porsche units take alook at Renegade Hybrids. They work mostly with Chevy engines, but they also have info about Fords. A good primer of sorts.
Also check Kennedy Engineering's web site. The 915 has been used in GT40s. That much is a fact. There is a guy in Nashville who has a DRB with a 915. Only issues,that I know of, was in converting it from cable clutch to hydraulic; as you can imagine, it took a little engineering and a couple of iterations, but I think it is working fine now. I hope to meet with him Thursday afternoon. If you'd like, I'll ask him about talking with you.
One thing about the 915, it is much cheaper to change the gears and diff in it than the G50. It does have a little lower torque rating, but our bretheren across the big pond have proven that this is not necessarily a show stopper at all!