I experienced the same phenomenon as i82much pointed out. When my sb350 was first built it could spin the 28ish tires over with relative ease with a turbo 350 and 3:07 gears. Fast forward to today with 35 in tires (Goodyear MT/R) and a 4:10 rear end it can barely get them to spin (Detroit locker) with less than adequate acceleration. Braking distance was also drastically increased. The price you pay for off road capability. . .
With that said a 383 the truck should behave more like it would if it had a big block so something is amiss. With big tires I would focus on how much torque the motor makes and when. As Carrol Shelby famously said “horsepower sells cars, torque wins races.” Or something like that. I also like this one:
https://images.cdn.circlesix.co/imag...6473fb4b93.jpg