If you try to do a rope rear main in the car, chances are nearly 100% that it will leak.
Packing the rear main has never worked for me because if it's old and leaking, the rope has deteriorated and packing new ends in it won't hold the oil back.
Pulling a new rope in with Chinese fingers damages the rope even when funnels are used, been there and done it.
I've never put a one piece Viton or Neoprene in a Pontiac, I've done others, but the crank does need to come out of the block, requiring pulling the engine. Two piece Viton/Neoprene needs to be silicone/glued in the block and left to set up for 24 hours with the main cap installed, before installing the crank.
Using the Best graphite rope seal would be best to pull the crank and fit the upper half into the block for a leak free install.
Just to sum it up, the engine should come out for best results no matter what seal you choose. Whoever you select to do the work should be intimately familiar with Pontiac rear main seals (most general service garages probably never even attempted to install a new main seal in a Pontiac).
Even someone familiar with them can have a leaker, a 24 hour test with the engine filled with oil and tipping the engine so the engine oil is against the seal minimizes the possibility of installing the engine and having it leak. Think of it a pre test before installing the engine back into the chassis.
Labor is going to be expensive, I would guess 12-15 hours depending upon labor rate $1000 and up.
My qualifications to post on the subject, 40+ years as a mechanic and have worked at Pontiac, Buick, and Olds dealerships, all of which used rope rear main seals. Owned my own garage which specialized in Pontiacs, but have worked on damn near anything automotive.
FWIW, if you came into my garage, I would insist upon pulling the engine, because I have tried and won some, and lost some trying to do them in the car. Buicks, Olds, and even chevy are much narrower blocks and the pan comes off and is able to be removed in the chassis. Pontiac is really tough to get off in chassis, I have done it twice, and for all the aggravation pulling the engine is easiest and you'll have a much better job resealing a Pontiac.
Good luck...……..
