Rear End Gears

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My 72 Chevy truck is a factory shortbed fleetside and originally was a 6 cylinder with turbo 350 transmission .When I got it ,it had been hotrodded and switched to a 350 V8 .It has been in the bodyshop for a frame off for almost 3 years now and is almost ready to come home but now has a new 350 crate motor in it .What rear gears were most likely in it from the factory and what would be the best for better gas mileage .I have not checked it to see what is currently in it ,it may the the original .