1/4 mile ET guesses

Since it seems to run fine and then falls off at the big end, i would suspect a fuel delivery issue. Weak fuel pump, clogged filter, kinked line, etc. Get a fuel pressure gauge and mount it out on the cowl where you can read it going down the track. With the 1/8 mile times you are running now , you could possibly get a 14.99 if you could get it to pull the full 1/4 without laying down. I bet someone has already put a mild cam in that motor. A stock 318 cam is usually pretty much done by 4500 rpm.