I don't have any experience with vortec or fastburn heads, but I would strongly caution against using 305 heads on a 350. If I'm not mistaken, 305 heads have a 56cc combustion chamber, versus 64 or 72cc's on the 350 heads. That's a 12.5% reduction in chamber volume, so on a 9:1 engine, it would give you 10.125:1. It sounds like a great way to make power, but without a piston dome and combustion chamber built for that kind of compression, detonation would be likely. Since stock 305 heads are almost universally from smog motors, it's a safe bet that the chamber wasn't built for high-compression. Adding to the problem is the fact that 305's have a much smaller bore than 350's,(3.74" vs. 4") so the chamber is built to fit the smaller hole. This results in a flat ledge surrounding the combustion chamber, which isn't terrible in and of itself, except it causes the chamber to be shaped even less optimally than the same heads on a 305.
Granted, this is all technical theory, and maybe I should have started out by saying, I know from experience it's a bad idea. A friend of mine and I stuck some 305 heads on his 350, and spent the rest of the time he had with the truck fiddling with the timing and filling it with premium gas, because that was the only way to keep it from pinging like mad.
And remember, free advice is worth every penny.
