The th400 is a good transmission, however first gear is fairly high and top gear is direct. What this means is the transmission was much better suited for a Cadillac than it is for a truck pulling a gooseneck.
So putting higher gears in the truck may suffer getting a load going, and keeping the 4.56 gears the truck will tach high and get poor economy on the road.
Changing transmissions may be a better option than changing rears. I would look into a built 700R4 transmission which would give you a lower first gear and an OD. I'm not sure if buick, pontiac, oldsmobile, or cadillac ever used the 700R4, so an adapter plate may be in order. I son't know what size tires you will be running, but the following table is my reccomendation for tires and gears with an OD transmission. This is not a law or anything, just a reccomendation.
28 inch tires with 3.73 gears
31 inch tires with 4.11 gears
35 inch tires with 4.56 gears
Most of these combinations will give you about 2200 rpm at 70 mph with an OD transmission. The gears or tires can be changed to push toward economy or work. These reccomendations are what I feel to be middle ground
