Do you know the rear seal size? I have built a lot of Pontiac engine and they all use a rope seal. Everything has been tried on them since the asbestos was removed from the rope 10 or so years ago. The new stuff just falls apart and is not worth the time or effert to put it in.
This just came out and really works good, so good infact, I had to replace one because it was so tight it would lock the engine solid after heating up. The second one was installed looser and has been working great for a year now.
http://www.dcimotorsports.com/PRODUCT%20-%20BESTgasket%20Pontiac%20Rear%20Main%20Rope%20Seal%20-%20350,%20389,%20400,%20421,%20428,%20455.html
It comes in two sizes so maybe one of them would fit your engine. The Stanley Steamer I worked on used a lot if this type of seal only smaller diameters. Most of what I used can from England, steam cars are very popular over there. Do a search for rope packing material, you just might find something that works.
