How to get 400 hp out of 1971 340 .30 over

It boils down to the sum of parts, going to need some head work to reach an honest 400.
It can be done with over the counter parts but you will need to open your wallet.
Most of the 340's with subpar factory heads are running well under 400.

I’ve heard a lot of small block guys say their car “should make 400 hp”. I think some folks need to recalibrate their denim dyno.

FYI OP, I have a similar build to yours but in a 360. Flat tops, RPM Intake, cam, and 750 DP. I made 413 hp / 430 ft lbs.