Dodge Camper Van w/318- switching 3-Speed to Overdrive transmission?

Hi all. This is my first time posting here. But it seems like the kind of place where I could get some good advice.

I recently bought a Dodge camper van with a 3-speed in it. And I'm curious about what is involved in putting in an overdrive transmission.

It seems like the three speed isn't going to be great for long hauls, at higher speeds.

And if I want to cruise at 70 miles an hour for a cross country trip, do I also need to replace the rear end?

Currently has a two barrel, which is not very responsive after 50 mph. I would like to upgrade the rig so it responds to gas, and cruises nicely.

I know the engine has plenty of power, but right now it feels like a laggard and I'm not sure if it's simply carburetion , or a combination of carburetion, and gearing in the rear end.

I guess my biggest question is is this a simple process of plug and play? Or is it going to require a bunch of modification.

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