trying to pull ponnies with little cash

If your running the original 7 1/4 rear it most likely has 2.76 gearing. It's not going to be a tire burner even with 400 HP with those gears. I would change the rear end first and then start with some bolt on parts. You can add 100 HP to a stock 318 with bolt on parts using stock heads. 300 HP is easy to come by 400 will take better heads or head work and a larger cam.