Here's my set up, My 383 will spin close to 6500rpm. 11:1 compression. The cam is just big enough (operating range is rated 1800-6000rpm) that launching the truck in 4HI by dumping the clutch with the 5speed, the 3.42 gears and the 33" tires brought the engine down pretty bad. My tires are the Dick Cepek Mud Countrys. Pretty knobby. Since the area I was launching from was fairly dry it was difficult to get the fronts and rears to spin. The truck weighed in at 4800lbs last I checked and I've now added another 250lb 6" lift kit. First gear in the 5speed was 4.01:1. First gear in the TH400 is going to be 2.48:1. With the added weight and the higher first gear I can kiss launching in 4HI with the TH400 and the 3.42 gears goodbye. It was difficult to shift the 5speed into second gear while going through the pit. The sticky mud tries to stop the tires the instant I hit the clutch. No matter how fast I tried to shift I just couldn't keep the rpms up. The synchros in the factory 5speed being old made it hard to shift fast anyway. I've eliminated the shifting problem with the TH400. I have a B&M Holeshot 2000rpm stall and a TransGo stage 2 shift kit. The 400 will slam through the gears fast and hard. So, my dilemma right now is the launch. According to the gear ratio calculators, I was able to attain about 40mph of wheel speed staying in first gear with the 5speed in 4HI. If the TH400 shifts out all the way to third gear I will attain 60mph at about 6000rrpm in 4LO with a much harder launch. This is all on paper right now. I haven't installed the tranny yet to test it. My concern is whether the tcase can stand the hp of the 383 in 4LO. I'm not sure how it manages the gear reduction. Additionally the input shaft of the tcase will be spinning at whatever rate the tailshaft of the TH400 produces in 3rd gear at 6000rpm. Something similar to running the truck at about 140mph in 2HI.
I guess I should probably just install new gears in the differentials and be done with it. Since I don't have any experience doing ring and pinions I'll have to pay someone else to do it and that's around 1500 from the quotes I've gotten. Between the 383, the lift kit, the wheels/tires and the tranny upgrade I've already got around 8000 invested in this thing over the last year and a half. I sure hope I get more than one pass out of this thing once it's finished. lol