You do not have to remove the crank when replacing the 1 piece or 2 piece rear main seal for the sbc 350.
On the 2 piece, remove the bearing cap, and with the help of someone turning the crank, use a small blunted object to push the top half of the seal through the other side to get a hold of and pull it out....Installing --it helps to turn the crank also to get the new seal to go into the top half...I believe the lips of the seal are towards the inside of the motor.
The 1 piece seal, which I have, you will need to move the tranny out of the way and remove the flywheel to get to the rear main seal.
There are 3 tabs that you can insert a flathead screwdriver into and either pry or twist the screwdriver a little at a time and the seal will pop out quite easily.
I used a 4" long, 1/2" extension and rubber mallet to tap the new seal back into place.
I started the motor, which hasn't been started for about 2 months, and let it warm up for a while. I'll check for leaks and if none found, then I'll finish installing the brakes, and start working on installing the 700r4 sometime this week.