1968 Dodge Dart 8 3/4 Chrysler vent leak

Hello this is a father son project car in the making, my dad refused to help me as I'm 19 years old and told me to ask the smart guys on here! We have done a mini tub job to this car so it has a 15inch wide tire in the rear and the axle was shortened to accommodate, HOWEVER SINCE THIS JOB the rear axle had a vent i provided in the pictures and it wont stop slowly leaking gear oil any thoughts how to fix this would be much appreciated!

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