What goodies do I need to build a Fast slant 6

You can get a /6 to run a lot better for cheap. but you should have realistic expectations. The easy stuff is mill the head about .060 to .080". Get a cam and lifters in the 268 duration and .450 lift or less. Get a factory 2 barrel intake and carb. Since you are running a 66 Valiant, you could use a 66 Commando Intermediate 2 1/2 pipe, muffler, tail pipe, and have a head pipe made to connect. That would transform your factory /6 but would not get you 300hp. It would be fun!