I suggest you get your trannie fixed first. This will cost some where between 2 - 3 thousand. This alone will make towing much more enjoyable and improve performance about as much as an EZ.
The problem is the OEM torque converter has a lot of slippage ( referred to as a slushbox) and the lockup clutch ( this is what locks when you are cruising) will not hold a lot of additional power. When this clutch starts to slip you rpms will go up and your speed will go down and then you will have to get it fixed.
A new TC will will have less slippage and enable you to "get the power to the ground", this lets you better utilize the power that you have. A new valve body will increase the holding power of the lockup clutches. There are other improvements that increase longevity and allow for future installation of an exhaust brake.
If you increase your power first ( as I did ) you will probably have to get the trannie fixed eventually and have to put up with the slushbox until then. I had an excuse, I didn't know any better because I hadn't heard about TDR and all the good advice available here. I hope you heed this advice and save yourself a lot of trouble.
I don't want to start another war but you can see in my signature that I have DTT upgrades and I feel they are the best. Do your homework and good luck.
Tom