I've dealt with the slant 6's, they are affected by their rpm limitations due to their long stroke and long connecting rods. I'm sure that with a good stall converter and gears you might be able to get some performance from them, but no matter how you look at it they are not known for their rpm capability.
The main thing holding /6 back is the head stock your lucky to get 200 hp out of it and fully ported around 300 hp, next is tiny bore a /6 makes a 273 bore huge in comparison. Gonna have to turn /6 fairly high for 300 hp.
