Help! No power to the wheels!

[piecesparts]

You guys have covered a lot here. I have not heard the obvious part, though. Is the tranny having issues with the bands inside it? You changed out the engine and connected it to the old tranny that was still in the truck. Did you pull the torque converter out of the tranny and if so, why? You could have disconnect it from the engine and lifted the engine without pulling the torque converter and it could have set int he frame waiting on the return of the engine.

If the engine is reving more than you think it should be for the transmission to work properly, then there is slippage in the tranny or there is slippage in the rear end. There was 4 quarts of fluid added to the tranny, so after warm up, what does the dipstick show for oil level? I believe to fill a tranny from a dry condition, it takes more tranny fluid, than just 4 quarts. The oil pan removed and nothing else will take 4 quarts, but there is oil in the torque converter and the valve body, also. My guess would be a total of about 10 quarts could be expected to fill everything to a point that the dipstick shows oil with the tranny warm and the engine running while oyu check it. Do not check it with the engine shut off.