The 390 from the factory had a 401 HP version as well as a later 335 HP GT version. Check out the specs on these engines. The stock exhaust was very restrictive on these engines and an upgrade was an immediately improvement.
As far as HP increases, it's really difficult to know without knowing which version (compression ratio, 2 or 4 barrel version, year) you're starting with.
Is it a two or four barrel (310/325 hp)Go with Edlebrock "performer"package Cylinder heads will get you 60hp alone.The package is Intake,Heads(pr)and Cam right around 105hp gain.If you can find a 427FE the heads and manifold will bolt right up.This is a great motor assembled correctly with the potential for huge torque.both sites below will send free catalogue and send E-Mail offers from time to time.What are you putting this motor in? I have a 64' Galaxie500xl with a massaged 390.The car weighs 2 tons and will spank many unsuspecting Mustangs and used police cars