Making Power 318
What I would do before I spent ONE RED CENT is to make DAMN SURE what's there is in the top optimum tune for performance. Run a compression test. This will tell you the general condition of the engine. Get out the timing light and see what kind of curve is in the distributor. Make sure the carburetor is tuned correctly. The thing just might be way out of tune.
THIS!!!
What is your feel for general engine condition? Does it "blow" oil smoke or use lots of oil? You MUST ^^as suggested^^ run a compression / leakdown test. Get/ borrow a timing light and see what the timing is doing. CHECK throttle is the carb fully opening, is the choke fully opening when warm?
Do you have a tach? Do you know at what RPM it shifts?
Also someone mentioned the rear axle. Determine the gear ratio, and post up your rear tire size. To determine the ratio, safely block the tires, jack up ONE rear, and put transmission in neutral, parking brake released. Attempt to turn the tire. If it will turn, you have NO limited slip. Turn the tire TWO exact turns, while counting the turns on the drive shaft. If it turns "lets say" 3 1/2 turns + a tiny bit more, you have 3.55 etc
If the tire will NOT turn you have a limited slip rear. Jack up BOTH rear tires and verify that they both turn with each other. Now turn either tire exactly ONE turn while counting the driveshaft.
There are MANY variables to performance let me give you an example.
When I had my 67 together, I obtained what turned out to be a VERY VERY low miles 318. VERY low miles. I could tell my looking at the cam/ lifters/ nylon timing sprocket. All I did was install a little larger cam, headers, 4bbl. The thing was VERY impressive even with stock heads and low compression. I admit I do have 3.7x gears which adds a bit of snap.