My AOD trans in my 91 GT would shift from 2nd to 3rd at 4,700 RPM. I thought something was wrong, but I read and was told they all do that. The 1-2-1 shift is the only way I could see your car getting into the 12's. When I 1-2-1 shifted the car, it was a beast!!!! It also had GT40 Heads, Cobra intake, RR's, E-cam, headers, H-pipe, cat-back, 4.10's. Suspension was all stock, never had it at the track, but on the street that thing flew on the 1-2-1 shift, otherwise in D, it was a DOG!!!!!
I'd also inspect your motor mounts. Previous cars (5-spds) that had poly mounts were stock engines and suspensions, but when I changed the mounts over, the car transfered weight better and faster and I could bark 3rd gear, which could NEVER be done before.
I'd also inspect your motor mounts. Previous cars (5-spds) that had poly mounts were stock engines and suspensions, but when I changed the mounts over, the car transfered weight better and faster and I could bark 3rd gear, which could NEVER be done before.