The difference between Drive and manual Low is the low-reverse clutch. It's applied in Low range for engine braking, and the low roller clutch is the only thing holding in 1st. gear drive. I'll second the suggestion that the screech is probably the roller clutch. It's a weak point on the 350. Every snowstorm around here, I could count on at least one trans failure from people "rocking" the car from reverse to drive to get unstuck. The car would come in with no movement in drive, but it would pull away from a stop in low range.
Shift kits are a big mistake. Just about every failed rear end I've encountered was in a car with a shift kit, usually with an elongated hole where the spider gear pivot shaft fits. For a better-performing TH 350, I usually machine the clutch pistons enough to add a fiber and steel plate to the assembly, and don't mess with the hydraulic circuits. Now, I'm sure it's time for all the experts out there to inform me I'm all wet!
Jerry
