what is the best overdrive transmission for a Duster

So I have a 73 Duster with a built 318 and a 8 ¾ 3.55 posi rear end. I cruise at 3000 at 60mph and the SOB gets 9.6mpg when I am really nice to it. haha so I need an overdrive. besides 5k for a Tremec what is the cheapest and best route to go without making new supports and totally reworking the mounts and car braces.

I know an 833 came in overdrive and 3 speed didn't it? I'd love a manual but what is the running price of a 833 overdrive.

Would a overdrive automatic bolt in easier? cheaper? or is there one that works for a Duster

-Thanks