You have to drop the oil pan. I don't think you can do that without lifting the engine. https://www.v8buick.com/index.php?threads/350-main-seal.337253/#post-2840163 (See post #9) https://www.v8buick.com/index.php?threads/rear-main-seal-replacement-instructions.115596/
I removed one motor mount bolt and loosened the other just had to raise the motor an inch or so. Helps to rotate the crank to clear the throws. Wasn’t a fun job, but it was successful!
It only takes 5 minutes to remove the long engine mount bolts. Then raise the engine up to do the job.
The last time I replaced the rear seal, the motor was on the engine stand. I turned the engine upside down & removed the pan. In order to install the block half of the seal, with some crank lube on it, required that I loosen the main caps & lift the crank a little. It was easy to install the other half of the seal on the cap. However the block inserts where a different story. My rear sell is again leaking a lot & I think we messed up the inserts. The motor is coming out again for that problem & to have the heads planed as I have a leak at at cyl #1. The instructions the T/A provides with the seal are quite good. Follow them.
Did anybody think to mention that if the engine has not been run for awhile it may be that the seal has just dried out? Good idea to run it for awhile and see if it quits leaking.
Actually, the car is driven regular - the the April - December 1 (?) at least once a week. Thru the winter, if the roads are clean, salt free, I try and drive twice a month.