How much lighter is the 904 ? Back in the 70's I blew 4 904 transmissions , 3 had kits and extra cooler on it , they just did not last . Put a 727 in and never had a problem again .
I believe their about 25 lbs. lighter if memory serves me. I noticed 904's didn't last good back in the 70's too and many guys swapped to a 727 and never had any trouble. The 727 is a much beefier trans. so naturally it'll take more abuse. I wonder if the reason 904's didn't last back then is because 1. a lot of guys didn't know the kickdown linkage also controlled the clutch pack pressure and when not adjusted right it'll trash the trans. pretty fast and 2. shift improver kits back then weren't up to par and caused just as many problems (if not more) than they helped. Torqueflites are very good transmissions but they have a couple things that have to be absolutely right or they won't live. One of those things is that in stock form your not supposed to manually shift them because they'll overlap on the 1-2 shift. You may not feel it but it's there and doing damage. The only torqueflites that were the exception to that rule were the ones used behind high perf. 340's, 440's and Hemi's.
Back in the late 80's to early 90's I installed 2 different brand shift improver kits in different torqueflites and they caused major 1-2 shift overlap. One vehicle was my truck I had to use everyday for work and I had to run it with the junk kit in it for about a month before I had time to take it back apart and put it back to stock. You wouldn't believe how much clutch/band material it had in the pan in just 1 month. I bet if I had left it in the trans. would have not lasted a year and it only had 33,000 miles on it when I put the kit in. Terrible, just terrible. That kit was labeled a Mopar Perf. kit and the other kit that overlapped was a B&M. It was at that time I studied up on how every part worked in a torqueflite so I could figure out why that happened.