239 intake duration at .050 should pull hard to 6000 rpm.

Higher than necessary valvespring tension costs HP.

With .484 lift there's quite a few economical choices for valvesprings out there. I'd go with something equivalent, but cheaper, to the old MP "249" spring for lift up to .510".

The key is to not just set them up to their "install height" but to have the right "closed" and "open" pressures as mentioned above.

Have you checked the top bushing of the distributor for any freeplay?

Got a fuel pressure gauge reading at WOT?