Van5150

Check your Trans Fluid level ,.,.

I run a 3600 Circle-D -- And had similar issue, I installed a new Trans Cooler -- the next day, had no power,. had to really rev it to go,, for giggles I checked level and it was around 2 quarts down.

midnight01

Another question not asked, what is the rpm range of the cam? you have a high range torque convertor that should be matched to the range of the camshaft as well at least from my experience. That's a lot of convertor. the gears are not helping but they should not be the real problem. you need to go with a street convertor, what you have is a drag convertor. You need about 500 pounds of torque to be able to launch like a madman with that convertor. Also 3800 stall is rating your powerband to be at the 5000rpm range roughly hence why i asked about the rpm range of the cam.. I also hope you have a better transmission cooler installed too. Your transmission is going to heat up pretty quickly. Go and get your car dynod and look where your powerband is. That will determine the stall speed you really need.

Bluefire

If you have the 2:73 gears, dump them. They're "the suck". I promise you if you put 3:42's in there, you will make a wet spot!!
My coupe came with 2:73's and i could barely break the tires loose.

Enter the 3:42's, H/C, and a 2800 Yank stall... then it was a smoke show!