You would be giving up about 100 cubic inches if you switched to the 6. Unless you went to a turbo your performance is going to suffer, especially in a truck that's heavy anyway.
Assuming you built the 250 you would have to make about 1.2 hp/ci to equal the 300 hp you have now. While that's pretty easy to do, you would have to sacrifice low end TQ and spin the engine higher to make that much hp from that size engine.
My vote would be to build a torquey 383 V8, and use your present TBI. It would handle the extra
~30 ci just by raising your fuel pressure and be a lot more fun to drive.
Ron
