rear mani seal

Slant 6 or V8?
Slants are much easier to change in the car. Drop the oil pan, remove the main seal retainer, remove the old seal, and reverse the process. You may have to loosen the main caps to get a little wiggle room but there's only 4 of them.
V8 is a little more complicated since it doesn't have a separate main seal retainer (it's part of the rear main cap), you have a lot less room to work in, and you have 5 main caps to loosen and an oil pump to remove.
Either way, plan on being on your back all day, losing skin, getting eyes full of dirt, and having oil drips in the face.
Or pull the engine.