It's not rocket science, the 727 book is a great help and the clearance suggested apply to A904 transmissions. I have used Griner and CRT Valve bodies, they work great.

Started doing my own 904's after a trans shop close by didn't even get me to the shed before they bent me over, so now I do my own and with help from John Cope and Rick Allison of A@A I now have no 904 issues. with the proper selection of part's and some oiling mods mine have given me good service life.. one thing about cheeta trans brake VB's is you need to up the line pressure from what they build them at and they will work good and give you a VB that won't burn your cluches up, found out the hard way.

Use the 727 and go slower so what I got sick of the 904s coming apart under power from Mild 360 in a pig heavy car There is a .15 diff in times I can tell you that but 22 years on a 727 and still runs like crazy. Normal maintenance and ATF 4 Mopar fluid. The 904s were good for 800 miles and 5 to 10 passes and then broken to point of nothing salvageable except valve body

.....that's my concern too Paul. That's why our Duster has a 727. With the completion of our W5 mill when my dad was wheeling the car, the last thing I wanted was for him to have to babysit a transmission.... so 727 it was. Kind of the same reason we chit canned the 8.75 for the D60. Just one more thing on the "set it.... and forget it" list.

The 904 we have was behind our 10.9/low 11 iron head 360. It was built by Chuck Lofgren and other than the Griner trans brake, nothing real extravagant. I think it would be interesting behind our current W8 mill, although an expensive test as I'd need a converter, driveshaft, and imho... a good input shaft. Maybe someday...

I ran 904's for years with great luck. Best move I made was going to a A-1 brake when I was running my 725 hp W7 motor. The T/A brake would every once in a while creep on me and the A-1 never did that. With the A-1 brake, we added a billet input shaft from A-1 and other than an additional clutch and good parts, it ran great This was in a 2650 Cuda running low nines. Also had the stock 2.45 gear set.