340 No low end power

Before you change anything: have you verified that the engine is, in fact a 340?
Have you done a compression check? Once you have verified the condition of the engine, you can proceed.
The phrases "fresh rebuild" and "built to six-pack specs" don't mean anything.
Come to think of it, I've seen several engines built to 6-pack and even 12-pack specs, those being 6-packs of Bud or Coors. They didn't run very well.