help with 64 dart rearend

I have a 64 dart gt with the slant 6 and the small 7.25 rear differential. I noticed a small leak on the differential 2 weeks ago, tightened the bolts, and it still seems to be leaking at the bottom. I am having trouble locating the gasket, does anyone have a part number or a part source? I want to take the car on a trip to Watkins Glenn race track this Summer, so I can show my wife's friend and Husband the car while he races his Porsche at the track. I want to make sure it makes the trip.