Have you verified the A500 has the low gear set? I know all over the internet you read all A500's had the low gear set but I rebuilt a 87 A500 for a buddy and it did not have the low gear set.
If your trying to piece together a low gear 904 you need everything from the sun shell/gear forward to make sure it'll all mate up. And verify that the output shaft is the same design/size where the back end of the input shaft mates up with it. I'm not sure on 904's I know on 727's there were a few designs.
BTW: have you checked out
www.wittrans.com for parts? They scrap out and refurb a lot of trans. parts and sell them at decent prices. You'd have some shipping charges but considering how much time and gas you could save you would probably be way ahead buying the parts you need there.
BTW2: you mention you want to replace your front planetary. JFYI you will definetly have to do that as it is part of the low gear set.
