I did this on my '67 2 years/4000 miles ago. Pull the engine. Very easy. And once it's out and on a stand, very easy to do the rear main seal without a total teardown. I pulled the mains and raised the crank at the rear, but didn't remove the crank from the engine. Checked the bearings, etc (77k on the rebuild at that time), all was fine, installed the BOP seal carefully with no issues. While I was at it, installed all new brass core plugs, timing cover gaskets, etc., and detailed the engine and engine bay. Actually, a pretty pleasant job. And, 2 years later, finally a dry undercarriage with NO oil leaks. I have a special tool that mounts to the timing cover to raise the engine so "in car" rear seals can be done, but I've BTDT, and it's several times the work of simply pulling the engine out of the car and doing it right.
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Jeff 
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