[Big Dave]

This is a doable project.

There used to be a cat around here that transplanted a 6.2 into a Cadillac Fleetwood of similar vintage. He couldn't shoehorn a turbo under the hood but for a pure MPG-mobile the NA engine will work just fine. I know he is active on the 62-65dieselpage.

Do yourself an immense favor and modify the car to use a manual transmission. As strange as it seems, these underpowered engines eat automatics like teenagers eat potato chips. I had a 6.2 in a pickup and it went through five 700 R4s and two TH400s over the course of 305,000 miles.

You are aiming to reduce engine speed at a given road speed and that means the TC doesn't get enough fluid flow to cool it. Expect spectacular failures (parts scattered along the road) at interstate speeds.

The stick eliminates that possibility, and also eliminates a layer of electronic complication.

The stick will easily gain you 2-3 MPG.

Check out the impalassforum. they have links and complete project guide on converting B-Bodies to use Tremec T-56 six-speed. A supplier called F2B makes a conversion kit.

With your gearing, the T-56 has too much overdrive. I'd recommend a Richmond Gear 4+1. It's top gear is 1:1 which should be about perfect.

You'll need to upgrade your soundproofing. the 6.2 is efficient but noisy.

I think you could realize a 38 MPG station wagon. Maybe more as the stick will let you aggressively hypermile.