[Bigdav160]

I think the short answer is "no easy swaps".

A popular swap is a New Venture 4500 (NV4500). GM and Dodge both used this transmission. You can search this forum to find more details. I think the NP205 might need to be changed to a TH400 version. The transmission is longer so there will be more modifications will be necessary.

I never heard of an overdrive unit in place of the bell housing. I'm surprised there's enough available input shaft length to drive it.

I've heard of people putting a gears vendor unit behind the NP205 but I do not know what's entailed.

edit, with the tranmission change you might be able to improve your mileage a couple miles a gallon. It's not enough for a lot of people.