340 Build

Like I said earlier, my guess is it’s running just fine....... and is just not what the OP was expecting.
If the cam is off a couple degrees, sure that would help if it was right..... same for the compression.
If it’s 9.5 instead of 10....... making it 10 would help....... but neither of those things is going to transform the motor into a totally different beast....... unless something is pretty far off.

As for the powerband..... for comparison:
340+.030, very mild bowl port 587’s w-2.02 intake(never flowed, but almost certainly less than 220cfm), std performer intake, Comp 268h([email protected]), 650dp carb, 1-5/8” headers....... peak tq@3800, peak hp@5200.
That’s with way less head, intake, and cam than the OP’s build.

If the valvetrain held on to say 6000-ish, I’d expect the OP’s build to get over 400hp.