Recommend a tranny shop in Phoenix?

I wish I knew of a decent shop Jeff (anywhere)

If you have a place to do it and some tools, I might be convinced to come down there and help you with it on any given Sunday. (more likely a Saturday morning)
Even if you don't want a "race" trans, you will want to make some minor mods to it.
Like,
Open up the 1-2 and the 2-3 oil orifices in the separator plate.
Block the accumulator.
Change the mainline pressure valve adjustment.
Locktite the front seal in place.
Take a little weight off the governor valve so it shifts a little later.

The problem is that in order to actually make it stronger you would need a few hard parts. (steel planetary set, 5 disc forward drum, band lever, kevlar band).
The 904's are pretty tough as they are though.

If nothing else, get the stage 2 shift kit for it.
This will give it more pressure to hold the clutchpacks and bands so they dont slip (300hp) and make it shift a ton better.
The kit will also make all the recommended changes I wrote above as part of the kit instructions.

A shift kit and a soft parts rebuild kit would be a really good call.

Soft parts rebuild would be clutch friction discs, band, the rubber lip seals inside the drums and all the outer seals. (gaskets and stuff are included)

I can do the whole thing in about 2 hrs. (If it's already out)

An inclosed area with lighting and a bench would be a must.