I would stick with the 904, I have one behind my 390 and have beat the snot out of it this summer with about 3300 miles. It was stock but I added a Transgo shift kit and it made the 1st to 2nd shifts so hard I actually thought I would snap a u-joint eventually but never did.
Mine started to not downshift automatically when stopping so my plan is to have it rebuilt this winter and continue to torture it. The 904 is pretty stout and should be able to handle all that a 390 stroker can dish out.