318 net hp is 150. Going from gross to net, they lost 80 hp (rated, not actual) with the same compression, carb, etc. as it had before. The 340 went from 275 supposedly gross to 240 net after losing two full points of CR, getting smaller exhaust valve and (at least compared to 68-70) worse passenger side exhaust manifold. That's less loss (on paper) than any big block that lost the same amount of CR. Actually, I've always thought the early 340 275 hp rating was net from the git go.
BUT - a 360 has 20 more cubes. And it's easier and cheaper to find one. If I'm rebuilding the motor anyway and didn't already have a 340, I'd look for a 360. Since I do have a virgin 340 block, if I build one more motor before I croak, it will be the 340 - but with a stroker crank, so the 20 less cubes in original form won't matter.
