1/16" bleed hole in the piston - in the correct place - for transbrake use is normal along with relocating the vent. Cracking the old style converter neck in the corners is normal and fixed by using the later neck and pump gears. The multiple spring pack kit in high gear is a good idea. A leak in the bellhousing is not the fault of the trans, buy they will drop some fluid if the crack in the converter neck is allowed to get too big. .

A mistake made putting any of it together, regardless the trans used, will result in a malfunction. It's not the fault of the 904, although admittedly, there can be more maintenance needed when put behind big horsepower. They work great when done correctly. Going back to a 727 is going backwards. Fix the little trans.

Is the TA valve body transbrake? LBA? Perhaps an up to date valve body would be a good idea.