[Sirrotica]

Take the engine out, if you try it in chassis you'll have a much higher chance of failure trying to do it upside down, rather than doing the work on an engine stand with the engine inverted. Plus it will drip oil on you for a week after you remove the pan, kinda like a form of torture.

Rear main in chassis, 99.9% chance it will leak, and the same odds you'll have to do it twice at least, ending up pulling the engine anyway. Pontiac shop manuals say it is possible to do the main seal in chassis, but I've never been successful doing it. I don't know anyone that has told me they did it successfully in chassis either. You really need the crank out of the block to at least have a fighting chance of having success.

If you decide to pull the engine, leave the transmission in the car. I have no idea why everyone pulls the 2 pieces out of the car together. Especially if you have limited space in a shop, leave the transmission in the car, tie it up to the cowl, and push the car outside. You don't need the transmission in your way while you're working on the engine, and you don't need a red sea of ATF (or gear lube if it's a stick car) all over the floor to clean up either.

I've tried this job multiple ways over half a century of wrenching, and the way I described it will give you the best possible odds at success. Work smart, take your time it's gonna be at least a weekend of 8 hours a day to complete. This is not a job to hurry, or try to shortcut. It's a miserable job no matter how many times you've done it, and disheartening if it leaks when you think you've fixed it.