3spd to 4spd OD trans

I can't answer all of your questions, but I can take a shot at a couple.
First, I don't think the converter will swap from the 904 to the A500. Seems like there is a difference in the pumps or the splines.....or something.
Second, it would probably be a deal IF its a good trans. Otherwise you've spent $200 for a core.
Third (or fifth) Actually, in order to get the trans in the proper position and not in a "tail down" attitude, you'll need to open up the trans tunnel some. You can just modify the crossmember and lower the trans enough to clear on top. But that's a pretty cheesie way to install it. It will cause the motor to "lean back" and will screw with your pinion angle