My truck has 3.73 gears and 265/75-16 tires. I lock the TCC in third at 30 mph at zero tps and increase to 75 mph at 56% tps. It doesn't effect towing since the TCC unlocks with increased throttle. I don't think the GM TCC linings are designed to handle much torque so I wouldn't want to keep it locked with a lot of throttle, particularly with a 383 like yours. I also unchecked the flag that unlocks the TCC during upshift. I found it annoying when the TCC would unlock to shift into 4th and then lock again. The upshift with the TCC locked is not harsh at all.
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