Nitrous on a 318.....how much will it take

Back in the 80's I put a 125 hp plate on a stock (except for a LD4B manifold and a 4bbl holley from a 440) 74 dart. Over 100,000 miles on it, 904 trans, 2.93 pegleg 7 1/4 rear. opened the single exhaust pipe just before the muffler, and put on a set of 8x22x13 M&H slicks. First time ever playing with nitrous. Took it to the track,made one baseline pass on motor, ran around a high 15 or low 16 sec. Turned on the nitrous, and hit the button right after the high gear shift (left it in drive), not impressed. Next pass tried hitting it at the 2nd gear shift, felt better. Next pass, just as my right foot hit the floor, off the line, hit the button, what a difference. Ran a 13.8
This was my wifes street car, and only time at the track, but kept the nitrous on the car for 4 years. Only took it off, when we gave the car to her niece.