If you go to Eatonbalancing.com (Teds company), he has a section on Y-blocks including tips on installing the rear neoprene seal.You can also do a search on this site for that info.  Use the search box at top of page.  Here's the link.

http://www.eatonbalancing.com/blog/2008/09/10/neoprene-rear-seal-installation-for-the-y-and-others/ 

  

I assume you mean the rope is still in the block side(theres no upper "cap").Take a sheetmetal screw and screw it into the end of the rope(carefully dont hit the crank).Grab the screw with a pair of vicegrips.If possible have someone rotate the motor in the direction you are pulling on to assist "turning" the rope out of the block.This is a trick I learned about 25 years ago when i was doing a rear main on a 289 Mustang.It works!

  

I got my 312 on the stand and loosened up the crank journals .

Removed the top rop seal and cap.

How do I remove the bottom rope seal and install the neoprene seal?

I think I can lift the crank and punch out the old rope, then slide in the new seal.

Please advise

Thanks,

Richard