Lower cost transmission options?

I'm currently going through swapping the drivetrain in my '70 Duster from 318/904/7.25 to 360/904/8.25. I was planning on having the original A-904 rebuilt with lots of goodies by A-R-T Automotive in Franktown, CO but have now come to the conclusion that it's out of my budget. It was going to get an A&A reverse-manual VB, A-999 low gear set, deep cast aluminum pan, PTC converter, heavy-duty everything else... Basically the works. I know this would be the "right" way to get a good performance trans behind my 360 but I just can't afford it, unless I wait three months to save up the cash (and that's aside from all the other stuff I still have to buy for my car). What should I do? Find a used one in good shape and pull it apart myself? I've never played around with transmissions so it would be my first experience. One important point, I do plan to convert to a manual tranny (4- or 5-speed) in the near future (few years?) so the trans I run now doesn't have to be totally bulletproof. But I would still like to have peace-of-mind that it won't blow on me a few months down the road.