Classic/Antique Car Repair: 54 Chevy pick up, rear main leak, neoprene seal, good luck jack

Question
(two part question) I have been battling with this leak for awhile. I have tried to replace the seal with a more modern neoprene seal but can't quite get the leak to go away. I recently replaced the seal, it does not leak while at idle, not a drop. It's not until I put it in gear and drive.  I'm concerned that something may be out of balance, are there other causes that may create a leak, or do I need to go back to the original rope style seal. Is there anyway to avoid pulling the motor for this repair to install a rope seal?Any tips or advice?

Answer
I am a Model A guy, but here is what I think about your prob.  Lay under the car and have someone rev the engine.  You may find the leak is coming from a split in the oil pan and it only leaks at high rpm.  I had this condition on a brand new car once.  Find yourself a chevy car club and join it.  They will know all about your engine.  I don't think you will have to pull the engine.  You should be able to get to everything you need.  Find an old timer and he will tell you exactly what to do.  Search the net for chevy sites and you should be OK.  If you bought a new seal, you must have some contact that can help you.  When you get a Model A, I can really help you.
Good luck
jack