While I have seen 2004Rs that have been built up to be really tough, they just simply are not rated for the weight capacity that a truck would see. even the S10s only used them on 4 cylinder models. It would make much more sence to use a trans that's already built for the application than dump alot of money into one that may not work.
Either way, the bellhousing issue does kill the idea of a late model trans. I kind of suspected that there was a bolt pattern issue, but couldn't remember if it was Pontiac or Olds that it applied to.
I'm assuming that you're looking to use a late model trans to get the overdrive, but is there any reason you're wanting an automatic? A manual overdrive may be easier to adapt, either using a 5 speed from a later truck, an MY6 from an '80s Chevy, or perhaps a standard 4 speed with either a Gear Vendors or Ranger overdrive.
