a 428 and a good rear end, tranny, and tires should get you close.

frame connectors, poly bushings and good shocks and brakes will help it corner. a dropped front helps too.

a frind of mine bought a 2nd gen with a 403 and a 4spd, factory a/c, 80,000 miles. i dont know what the rear end was, but we put headers on it, and i bent up some 2.5" duals with flowmasters for him. he came back from the track with 13.8 on his windshield, and told me he smoked the tires thru first gear. this was a one owner, previously unmolested '79.

13 seconds is very fast. i have a feeling we would've had to do a hell of a lot more to make it a twelve, but not too much more to get a low 13.