Why not a built 200-4R? Roughly same external dimensions as a TH350, and can be built to handle a lot of power. You're in SoCal, so you're not far from some of the most well-known 200-4R builders in the nation. There's even a book out there on how to build up your own. I know Thomas (Bomber '67) has/had one in his 65 El Camino with a 496 that I think was pushing over 600HP