I had a problem with this as well even after using Teflon tape. I went and got a 3/4 inch NPT pipe tap and cleaned up the threads as best I could. I used ARP thread sealer and it fixed it. ARP thread sealer or a high qualer sealer is mandatory IMO if you have weak thread deptness like I had.

1970 challenger T/A 4sp. Go mango http://rides.webshots.com/photo/2052639870046004729EwHOJY