differences in 904's?

Sounds like what was called an A998. The 5 disk front clutch was common with the V-8 versions back to the early 70's. If you have this, you have a good core. Later trannys (A999) also had a lower gearset which is worth looking for if you have access to later units (junkyards have lots of Jeep Cherokees). Order the extra parts to do the front drum right. Various ring and washer thicknesses are available to properly clearance various assemblies.

Stock stall speed for a 904 was in the 1400-1600 RPM range, low enough. If you couldn't brake-stall past 800 you had a problem.