Getting power out of a 63 /6?

Hello, i have a 63 dart with a 225 /6 in it thats bone stock. I would like to get more power out of it so it could be better on the street. I drive it everyday so fuel economy is kinda important lol. I have headers for it and an offy 4 barrel intake manifold too. The carb i plan on using is a holley 390. Right now it still has the single barrel and stock exhaust and everything. What power could i see from these mods and what other mods are good as a starting point?

Thanks, Phil