Slant six rear main seal

I've done quite a few in car

the only way I would attempt to do a rear seal with the engine in the car would be if I had a car lift, like they
would have in a dealer maintenance bay,, then you could possibly get it done in 4 hrs, like the flat rate book lists.
no way I would do that laying on my back under the car,,,
without a lift, pull the motor, it would be quicker with a better probability of success.

another thought is a '72 probably had the rope style seal, you may not find those available today, you will find the two piece hard rubber seal, those take the same seal carrier and work fine.