They didn't come with stock 9-1 pistons. Factory compression on most LA 360's would barely cross 8 to 1. Yes, 400 hp can be made with HP manifolds. I assume your talking the early year (1968) 340 manifolds. They cost $$. 400 hp can be made with lesser expensive manifolds, but it's harder.
The Op has asked for recommendations. Please post yours. Just throwing out "go for 400 horse" isn't much help. Post a recipe, because I too would be interested. Give the pistons you would use to get your 400 hp, your cam, your heads (and work done to them), what exhaust manifolds, and where it would make it's power (rpm range). Will it work with 3.23 gears? The op probably doesn't want a high stall either.
