If you're not racing, there's nothing wrong with the 904's. They weigh less, and have less drivetrain loss than a 727. In fact, quite a few racers use them because they're more efficient- although they do have to be built up for that.
If you're planning for big horsepower down the road, getting a 727 may not be a bad idea. But if you're going to stay under 450 hp, a good rebuild and replacing a couple of key components will dramatically improve the 904.
But just because it "wouldn't shift right" doesn't mean you need a rebuild. More than likely it just needs fluid, or maybe a minor band adjustment. I was having shifting issues with mine, I thought I was in trouble. I dropped the pan, replaced the filter and fluid (both of which looked like hell), and viola, it shifted fine. Just because it doesn't have a drain plug doesn't mean the fluid doesn't need to be changed.
