I have a 1997 International 4700 Navistar. It has the 6 speed Allison Trani. Does anybody know where I could have this thing re-built to hold more power and torque?

I have chipped it and am now also running a propane injection and am putting out 150hp/ 300 ftlbs of torque more than the truck came with. I tow a 35000lbs set-up. The traini is slipping in 1st and 2nd and I know it is just a matter of time.

I would hate to spend money on a stock Allison and have it go on me right away.

Thanks for any suggestions