The Rear Main Seal is an age old problem with the "Rope" type seals used on the Early Birds. Rope seals are designed to weep oil and the installation must be made with the crank out and it's a matter of technique to do it right and it will eventually leak. The current rope seals are not made with the asbestos material) that the originals were and tend to leak sooner.

There are several options you have.

1.)There is a rope seal available for a Jaguar that still is still made with asbestos and will hold up longer $24.50 at COSCO.

2.)The Chrysler 318 V-8 has a elastomer seal, Felpro part number 40245 for $7.50 (thanks to Fred Jung), which has been used successfully. It has been reported that these work but the crush is higher than design and they wear out over time.

3.)Finally you can use the 292 elastomer seal but that requires the machining of the crank seal surface to the diameter of the 292 engine, $300+.

It has been suggested that for $7.50 go with the Mopar and change it as required when it wears out. I too am struggling with which way to go on my restoration and I'm leaning towards the Mopar seal.