Help me shave off 3 tenths in the 1/4

what rpm do you trap at?
what tire diameter are you using? 26 or 27" diameter max for 3.91
what altitude are you at?
518 tranny is a hp hog, heavy rotating mass. If you are running a stock high stall 2800 get a new one.
run as little fuel as possible, each gallon weighs 6 pounds.