42RH in 68 Dart advice/request

I'm about to open up an a500 that was originally from a 91 318 Dakota to freshen up.
The rest of that truck is long long gone, it's metal probably recycled into a couple of Kia's now a days. This trans is being considered for a beefed /6 in a d150. Yes I have the means to put a SB trans behind a/6.

I am just finishing up a 46re out of a Durango first.
I already have the "master kit" (with clutches and steels) for the a500, now I'm looking for heavier duty bands and the billet servos as I find extra money (this is for a truck that I have apart in other ways so no hurry to get this trans done.
I hear of guys bragging about how great the 904 is, etc and how they can be "built up to take some serious power" as I have never had much luck with them in low-po, daily driver duty let alone a truck or HP application. But although I hear constantly on forums like this one how "great" they are and how they can be built up, somehow I'm not seeing as much available as for the 727/518 and what I am seeing appears to cost more.
The last a500 I did was in the 99 Dakota I used to have, it seems like back then things were more available. I put the billet accumulator and front servo into that one when I had that one apart. That was the extent of what I was wanting to do with that one to beef it up. Maybe stuff just isn't "as" available anymore like so many other parts?
Mainly looking for kevlar bands and a rear servo with the dual sealing rings like I put in the 46, only seeing billet disc with single sealing ring available for the lighter trans, though. Not the whole, beefed up servo "assembly" as I found for the heavier trans. The accumulator is the same between them and I'm able to see where to get the heavier, billet front servo as I did on that other one a few years ago.