Typically this vintage of Chevy came with the 2.73 or 2.93 gears, especially if it was a diesel from the factory. 3.23 gears were used alot in the trailer towing package but never have never seen a diesel with the towing option. If the 5.7 was ordered, the manual states no more then 1,000 lbs.
The Buick I converted started out life with 3.23 gears but when I tow with it, I use only 3rd gear. The 3.23 gears make it hard to maintain 60 MPH (noise more then the ability). Have since gone with the 2.93 but they are a little to tall when in 4th (1,650 at 60 MPH). There are a bunch of options available for these diffs from the aftermarket, but if you are using 15 in. tires, I wouldn't go higher the 3.23 or 3.43 with the 200.
Bill
91 Buick Roadmaster/Avant 6.2 NA conversion (gone but not forgotten)
94 Cadillac Fleetwood (sold)
08 Aerolight 23TT
06 Vortec Max Silverado CC SB (sold)
10 Avalanche (electronic quagmire but love the truck)