727 vs. 904

The tranny is in a going to school and work truck. No more than 15-20 miles at a time. It's starting to slip. The original tranny was a manual that the previous owner converted over. I have no idea what it came out of, or how many miles it has on it. I've got access to a few good /6 904's and was wondering what difference the 904 would make if I put one in the truck. I've heard that they have a problem handling the truck load due to the weight and rear ends.