I guess that if a guy really wanted to make the 351 go faster he could weld his own manifold. Make one that would take a big squarebore.
Jonathan's right on the rpm issue. I drove a lot of 2, 5 &10 ton trucks with the v6 in the 70's. Lots of raw torque, but not as much in the top end as needed.
The 478 had a lot more power, but it red-lined at 3250 and would not last very long at that speed. one truck we had was geared to 62 mph tops, and in 100,000 miles we blew 4 engines since it had been declared a road truck. It was a '73 model that was retired in '75. The last engine in that truck left a piece of block on the road that was as big as the palm of my XL hand!
The only good way to get more rpm and power out of the v6 is to pull it and drop in a big block chevy.
