I have an original 68 383 Automatic Formula S fastback car. I put a 268 comp cams in it when it was a daily driver. Not sure why 400 ft lb of torque would not be enough for you. Ran factory 72 400 intake and TQ for years on a build similar to what you want to do. 383 donor from a Newport, honed with torque plate, just to clean up the cylinders, Speed Pro file to fit rings, 906 heads, great valve job, cleaned up the bowls, windage tray, 402 pan, hp,hv oil pump, MP distributor kit with a chrome box. No lack of low end, definately would scream if called on. It would pull 16 to 18 mpg cruising highway. I always felt the original HP cam was better. Getting ready to redo the original 383, this time I went with a custom Racer Brown mechanical cam.