Sweet! I've done more than my share of lurking in the wagon section of the
impala SS forum. I was looking for one a few years ago when I needed to replace a car with something that could do some towing. Unfortunately I found a Tacoma first.
At the time I was looking those wagons seem like the perfect vehicle: room for an army, a ton of cargo (4x8 sheets of plywood!), or any combination of the two. Towing capacity of a 1/2 ton truck, but better mileage and more passenger and covered (& locked) cargo room. Only downside is the bad mileage when you're not using it to capacity.
That forum has a number of people with T-56 (6speed) transmissions, and there was one guy who
put in a 6.2 diesel
Around town due to all the weight it'll be drinking it down fast. If I recall, the older ones with the tbi motor responded a bit better to mileage mods than the lt1's, but for an all highway run, 25mpg should be a piece of cake, 27 shouldn't be too hard and with a few more mods and/or slower speed 30 ought to be within reach.