A518 46RH rebuild

I took out a 904 (998?/999?) from an 88 D100 I had back 30 years ago and put in a "518" (46rh) in its place.
88 was the first year of the Mopar rwd OD tranny's but I don't remember if it was a "mid year thing" or if you had the option when you went to buy a new truck that year but my particular truck was originally a 3 speed auto that I changed to a 4sp/OD auto.
I remember having to do something with that rear mount but it's been too long since for me to remember. I know that I didn't cut and splice the cross member. Maybe the cross member was the same and I just had to change the mount and add the 3 bolt adapter plate to the output shaft housing? Like I said I have CRS.
I'm considering doing the same thing to my current 85 d150 and am planning on going to the junkyard to find a cross member from an 89-93 d150 to take care of it this time.
Like I said / considering options, / it's a /6 truck, and the /6 is staying for the foreseeable future.
It's currently a 727, that was factory trans for this rig, I have a choice between that, the a500 and an 833OD, all of which I have right here.
One of the things that bugs me is that I've put 727s in in place of 904s and a518s in place of a500, but never the other way around.
And being that the 500 is a904 basically in the front half it seems like id be going backwards.
And yes I have the means to mount a SB trans behind the/6.......