I believe stock ported j heads. As I mentioned, I'm no builder, but a stock 340 put out almost same #'s the OP initially had. I think they "flowed" 255 @.600.
To me i would question the quality of the build OR defective parts.
There's porting and there's proper porting.
Heads in the wrong hands can be ruined.
I don't know what the op has for heads, but the big hp gain from an intake and carburetor change tells me it's wanting to breathe.