The LT1 is a good, reliable engine that will not have to work itself to death moving that big wagon around. The 4.3s are gutless in a sedan, I can only imagine how bad one would be in a wagon. That, and with a 2:56 axle ratio and the weight of 8 passengers or a trailer, the wagon will need all the help it can get to get rolling or go up hills. You'd have to work the smaller engine so much harder to overcome the gearing and mass of the vehicle that I can't see it having any real benefit over the LT1. If anything, depending on how much towing you plan to do or if you drive in a lot of stop & go traffic, I'd look into a rear axle ratio swap. 2:56s are great - if you are always cruising at highway speeds on flat ground. But they are a lot of gear to overcome from a stop, and with the weight of a trailer behind an already heavy car, they will put a lot of wear on your transmission as it works against them to get all that mass rolling or get it up a hill.
My '96 Fleetwood had a factory trailer package, and that included a limited slip differential with 3:42 gears, there was also another towing package with 2:93 gears. You might be able to find a Caprice/Roadmaster/Fleetwood with a trailer package at a wreckers and score the complete rear axle out of it for cheap. Just grab the PCM out of the trailer package vehicle too!
