In stock form the 307's were a bit of a turd, but built right, they can be a decent performing engine. Check out this build:
http://www.67-72chevytrucks.com/vboa...d.php?t=429865
Now, I don't know how well the 416 heads would work on a 307. Unless you can hear this engine run, I would shy away from it even if it is half the cost of a 350 GM crate engine.