Rear end cover

I just recently had new gears installed in my 8 1/4 and the cover keeps on leaking. He put a new gasket on that came with the kit, but the chiltons manual I have states that after 1972 they didn't use a gasket for the cover. It's a 1974 duster. The leak keeps getting worse and all the bolts are tight. At the 500mile interval i'll change the oil as it states in the yukon manual. When I re-install the rear cover should I use a gasket or just put on sealer like it states in the chiltons manual? Anyone been through this before. Any info is appreciated. Thanks
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