RE: 727 vs 904
There were a few derivatives between the 727 and the 545RFE.
Ma Mopar's first overdrive was the a-518 which was more closer to the 904 than the 727. If there were a choice between the 904 and the 727, the vote goes to the 727, hands down. The 904 was intended as the 'light-duty' transmission for the slant and the LA, and replacement to the 2 speed powerflite engines when it was introduced in 1962.
The Aluminum body 727 was introduced in 1960-61 to replace the A-466 Cast Iron TF. They were originally cable-shifted, and had a ball and trunnion connection to the driveshaft. The '60-'61 models had the emergency brake at the end of the tailshaft. The '62-'63 727s still had the ball and trunnion connection, but the E-brake was moved to the rear axle.
The cable-shift was changed in '65 to the linkage that we all know of now.
The horsepower ratings for the R/B series engines weren't as high in the late seventies and early eighties, and the 904 was reegineered with an overdrive as the A-518. The 727 was retinkered, and the small block bellhousing added to the trucks and vans, and I want to say it was called the 999. I remember my Grampa's '85 Dodge Van had that transmission...It looked like a 727, walked like a 727, quacked like a 727, but it was a 999.
As far as using the 904, the A-518 would be a better bet, as it is stronger and was designed with the 360 in mind.
As far as I know, the 545RFE has little or nothing in common with the 727, but I could be wrong...