Later model 3.8 swap in an "A" body

Over 20 mpg is going to be tough to pull off and still drive quickly. Got a 2008 Gm car with the 3.5 and I get only 21 mpg on it. Now I got a lead foot around town and go 80 to 90 on the highway if traffic is light.

If you were to do 55 mph all day, sure the mpg would improve.

I would get a custom grind cam if your going for mileage on a V8, the stock cam might be the cheapest but it can't be the best.