Good job. Always liked the boxy GMs. If your looking for mileage, keep the Buick but get a better set of gears for the rear end. These engines are best between 1,800 and 2,000 RPMs.
Did a very similar conversion on your Buick's younger sister (91 Buick Roadmaster wagon). With 3.23 gears and a 700R4 was able to get in the high 30s (broke 40MPG on one trip) with an average of 37 MPG (IMP) or 7.5 litres/100km.
A conversion on a 86 Cadillac Fleetwood I did with an Oldsmobile 5.7L diesel would consistantly get 40MPG, when it ran. Took me a few years to get the engine to be reliable.
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)