This is one of those jobs that's so labor intensive that it really pays to do it right. You could replace the rear main with a stock rope seal, but they don't contain asbestos anymore and it will most likely start leaking again. That said, I have a rope seal in my '68 that was installed in '98 and it still seals well. Next time I do it (and there will be a next time), I'm going to go with a
BOP Engineering neoprene rear main seal. Search the forums over at performanceyears.com and you'll get lots of feedback on the seals. They work well, but it's imperative to follow the installation instructions to the hilt otherwise it will leak like crazy.
If you decide you want to stick with the rope seal, here's a link to the
'68 service manual that should be of some help. Specifically, here's the page on
replacing the rear main seal.
Hope this helps!.