I had a 'bad' '73 Monte Carlo (4200 Lb car) with a built 350/350 combo. Put a shift kit in the TH350 and never had a problem.

I was 17 and broke the joints in the drive shaft, broke the differential gears (on separate occasions), and the transmission just hung in there. Gettin' rubber changing from 1st to 2nd was cool also .............

Shift Kit!

Good Luck, Kitz