To get another 100hp out of the 350's, you are probably going to spend around 15K by the time you buy heads, cams, lifters, rocker arms, pushrods, gaskets, exhaust and have dyno tuned. MIGHT pick up 10mph. Is it worth it to you? That is all that matters.

I did a pretty good 383 and spent I think around $5k. I can't bear to add up everything to be certain.
Previous question about drives is important too. Alpha drives aren't rated above the oomph of a 350. I have an alpha and I baby it cause my pockets aren't too deep. No wide open holeshots. No full power till on plane and moving well.