I've been running mine that way for several years now, without a problem from the three speed... well, except breaking the flex plate, but dad did that once before when the truck was totally stock and under warranty. The truck runs great with the mods, but as cerberusiam said, you'll really feel the weak link, the torque converter. I tow with mine on occasion; race car, trailer, tires, tools, gas cans, etc. , etc. I never have had a trans problem, and I worry about overheating it much more than I ever worry about trans failure.
Hope this helps.
Jim