I've never seen a TSP camshaft that would not lose a ton of torque, even their small cams have too much lift and duration IMO. Consider now in doing the gear swap, you are compounding... no, there's a better word, compensating for poor cam selection with the gears. When they say they are for trucks, think regular cab, short bed; about 4,500 pounds. Ours are easily 1500 pounds heavier.
The one thing our 6-speed trucks should never have a problem with is starting off in first, that's why they even let us start out in 2nd when its slippery out. With 3.42 gears, our 1st gear ratio is 13.77:1. This is even better than a Duramax with their 6-Speed Allison and 3.73 gears!
My advice, don't touch the gearing yet, dump that camshaft for an L33 grind or something slightly bigger from Cam Motion, Crane/Vinci or BTRs Truck Torque Cam.
Tell me about the heads. Stock heads or aftermarket? Chamber size and compression? Any work done to them? Porting can also rob you of torque or at least move it up higher into the RPM band.
Did you do before and after dyno runs? Any passes at the drag strip, before and after?
For comparison, mine puts down 307.52 RWHP and, if I calculated the torque right using RPMs at Trap Speed, she makes 337.1 RWTQ. Burning E85 @ 73% on stock ethanol tables, engine and trans tuned by BlackBear, Magnaflow cats and Y-pipe, no other mods.