I have a small leak from the rear main seal on my early 70's big block. I will be doing an auto to 5speed conversion, so everything should be cleared out back there. How hard would it be to replace that seal, remove the oil pan, flip the motor and all that? I understand that there are one piece and two piece seals, I have no idea what mine is.
Honestly, I'd pull it. Since you're doing the 5 speed swap, it's going to be that much easier getting the bellhousing dialed in correctly with the engine out of the car. Not to mention getting the pan off and back on good, with no leaks. Drop the engine back in with the clutch and bellhousing on and dialed in, and you're almost done.
Yikes, I knew this wouldn't be fun... I am not like you guys with engine hoists and stuff. I did disc brakes and set up a rear end, but I don't feel like yanking out the motor. I can do like I did on my 92 F-250 that has had a rear main leak since new: Take two paper towels, roll them into a tight cylinder, and stuff them between the scattershield front plate and the oil pan with a screwdriver. Replace the papertowels each oil change. Done.
Pull the engine, follow the instructions to the letter using the "shoehorn" supplied in the kit. Don't hurry. If you damage the seal buy another one. Don't gamble that it won't leak because it will. While it is out replace the VC gaskets too. Also give you a good chance to examine the intake gasket.
Chevelles are easy to work on compared to smog laden newer cars.