390s are good tough engines, but because of how old the motor is and never been rebuilt, i would go the rebuild method..... you've got a few options when you do this, you can stroke her out, use flat top pistons, install a good matched roller cam, 4bbl aluminum intake(cuts about 50lbs of weight off of a 390), you could add aluminum heads or just work out the ones you've got, costs are almost even when fully doing a set of cast irons and buying new aluminum heads (the aluminum will still give better performance so thats how i would go) with about 5-8k of money budgetted in your motor you could easily hit and even surpass 400hp