Juggernaut,
If the rear seal is leaking, it would have to be really bad to show up on the dipstick. Your delima doesn't sound like a rear seal leaking, but more like an adjustment problem. You should be able to see the
oil if the seal is leaking.
Try to adjust the clutch where there is no pressure on the clutch fork at all when the clutch pedal is at rest.
Cobras are not notoriosly hard on clutches mainly because of their light weight. A generic clutch should work well behind a basic 289 ... at least for a while.
Good luck!
Jody