I feel that I have better answers to your question as I went through the exact same thing on a '62 235.
The old rope seals are very old technology. The reason your engine likely has a neoprene seals is that they are much less likely to seep and leak than the rope style, and will hold up better. You can get a neoprene seal meant to reaplce the original rope seal for exactly this reason.
While I know all the books show that you need 0.001" to 0.003" clearance on the rear main, but I had 0.003" clearance and siezed the rear main 2 times. I took it to an old engine builder and we looked it over, and he told me to get a shim kit, and I did. We shimmed out the rear main further and it worked well. If it's just the rear main that's leaking, focuse on that, but don't take shims out as you could be causing issues with the bearing.
