The rope type rear main seal on Pontiacs really sucks -or to be more precise- blows. If there is an aftermarket seal available now- definitely get it- anything would be better than rope.

Mine leaked right after I did a rebuild, and burned out my clutch by the time I got to it. I replaced it with the engine in the car. There is a tool you can buy to replace rear mains with the engine and crank in the car. I think its called a "Sneaky Pete", by Lisle. It worked ok, as I remember. I was very careful to trim the rope very flush, then I put a ton of permatex black everywhere I could. That did the trick.

As I recall, if I had to do it over again I would have removed the engine. There are few things worse than lying on your back for 6 hours while oil is dripping in your eyes. Then you can't sleep that night because the muscle cramps set in from keeping your arms above your head for 6 hours.
Its true that you have to disconnect so much that the engine is 80% out anyway.
Take it out- you'll save yourself a lot of pain, and you will be able to clean and fix some other things while you're at it.