Transmission Options?

I have a '70 Duster with a 318 small block in need of a transmission. The goal for this project is to produce a DD / weekend warrior setup, so I'm pretty set on an auto with at least five gears to keep cruising RPM down.

I've done plenty of research into the GearVendors Over/Under-Drive aux gearbox. I'd love to have one, but it costs almost as much as the car itself and I'd have to get my leaky old 904 rebuilt.

Any way to pair a late model 5-speed like the W5A580 (NAG1) or 545RFE to a small block and control it?

Any Ford or GM transmissions that would work? I saw a 6-speed valve body for the 4L80 (if I recall correctly). Don't know that I'd want to go this route because it'd make me happier to stay all Chrysler, but if I can spend less and get exceptional performance without using 100% Mopar I'll just get over it.

Bear in mind I'm still a college student and I don't have an exorbitant amount of cash to drop on a transmission or auxiliary gearbox. I'd like to try and keep it under $2000 at the most.

Any guidance or insight would be much appreciated.