I would seriously consider the 5.3L swap. The 3800 is ok, my SC3800 has got 33.5 highway (nightime, 70 mph, 50F temps, no traffic, careful driving) on a round trip. It does ok for a wrong wheel drive 3800 lb car and 2.93 gears.
But a low mileage 5.3L from car-part.com is really really hard to beat. The 5.3L's will do mid to high 20's easily mpg with OD and a truck intake and taller gears. And you have 300+ hp on tap when you nail it. The 3800 is FAR less refined of an engine. It is still a buzzy 90 degree V6, it won't change that. A 60 degree V6 is smoother (by nature). But the GM 60 degree V6's suck on power. Even the newer 3.5L and 3.9L are lame.
The SC3800 can go nicely, you will want that one over a NA 3800. But the 5.3L is cheap, very very plentiful and FAR more reliable.