Just to follow up on a few things. As I mentioned above, the wide 5 clutch drums can only be found in an A999 (both lockup or non). The A998 has a 4 clutch drum but I wouldn't bother finding one.
A little over ten years ago I wrote up an 'article' for lack of a better term on how to identify the internal parts of an A999. Since you are starting with a later unit you might want to look through it and verify what you have will work with non-lockup parts which sounds like what you're trying to do.
How to ID a non-lockup A999 transmission with pics
The Moparts thread below is older than mine but it's where I got some of the information from for my thread. There is very specific info in it about what parts you need to make the non-lockup 5 clutch drum work. If you don't get much from my article you should definitely read this one carefully.
https://board.moparts.org/ubbthreads/ubbthreads.php/topics/24397.html
Everything you never wanted to know about 904s
I mentioned it already but
I do recommend the A&A race case. It's modified for several things the factory units don't have - first is a rear external vent. I use one of the plastic mushroom looking ones from brand x applications but you could also get an npt-to-AN adapter and run a puke line to a catch can or something. The rear vent is a tight fit to the floor pan so you my have to create a clearance dimple above it to prevent smashing it like I did. And be aware that when you relocate the vent to the rear of the case you have to plug the one in the front of the pump.
The race case is also welded and drilled just above the driver's side rail for an external pressure adjustment port (so you don't have to remove the pan). There are ports for temp and pressure gauges on the other side. There is a cutout at the bottom of the bell for easier converter bolt access. It's a nice upgrade for under $300.
I built my trans around the same time as I wrote the article. A&A sold me 6 thin clutch discs for the forward drum and I think 5 for the rear. Other than the other internal parts I mentioned already, it's nothing special. When I finished it I took to a shop that had a transmission dyno. They ran it and got it to shift and they said it was good to go. Unfortunately I never had it in my car because it's been apart since then but I can't wait to eventually run it through it's paces.
Only thing I've changed since then is the pan. I went with a Moroso fabricated pan because it has provisions for a temp sensor. The temp data gets fed into my EFI ecu which turns on the trans cooler fan at a given temp. It's also way lighter than the cast Summit one I had before.