swapping 5speed to Auto for race purposes only

You will do this once, and realize it's way too much work. And expensive. 904 trans is light, uses less horsepower, and can be built to easily handle 400 hp. The flexplate should be SFI certified, like a B&M. Also has to have the right balance for your engine. For a converter, you should contact a quality company like Dynamic converters. They will ask fo a lot of info before giving a recomendation. Good converters are not cheap. Driveshaft will probably be the wrong length. Slip yoke, cooling lines, trans cooler, shifter, etc. Good luck.