Bigblock is correct. I recently replaced my rear main seal with the Felpro kit. If I recall correctly, you fill the gaps with rtv silicone, then push 2 pipe cleaners into the gaps. My old seal was a rope type and a pain to get out. If you haven't come across this yet, this thread has some excellent info, including replacement of the rear main seal on post #28. I reference this thread all of the time!
http://67-72chevytrucks.com/vboard/s...hlight=gmc+305