Upgrades to my stock 273 67 Dart GT

You could spend a bunch of money and time buying various parts OR you could just have your heads ported and get more HP. Just find a set of 308s and clean up the bowels / valve job / backcut the intake valves . .
If the engine is original it’s probably close to needing one anyway . And you would have a solid base to build from which will complement any future upgrade . Then that 4 barrel intake will make a difference.