I'm about to put an offy/holley 390 on my 81's 300. If I like how it runs and the milage is good enough, I may swap a 300/T18 into my '77. But I don't plan on towing anything bigger than a bike trailer on the highway, and I won't be cruising over 55-60. I do plan on building a slide-in camper with the best aerodynamics I can provide.
A C6 will cost you @1.5mpg. C4 is more efficient but not nearly as strong. If you swap, consider keeping the np435.
