Yes, you will have to drain the oil and remove the oil pan. It helps if you can get a "sneeky pete" that is a tool to remove the old seal and help pull in the new seal. With the tansmission out and with the rear main bearing cap removed, loosen the pther main bearing caps to let the crank shaft down a bit. This will help get some space for the new seal to go around the crank shaft. There is a "Tech Tip" on this site that will help. It is a tough job, but it can be done. Important! There may be shims on the main bearing caps. Keep track of them and put them back exactly where they came from. Happy wenching. Clean every thing up and reassemble. Follow the Tech Tip and it will seal.
This might be helpful:
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0617.htm
