I'm not sure what they call it. The seal for the front windshield and the rear window.Question is what size 5/16 or 3/8. I thought the 3/8 was front and the 5/16 for the rear just want to see if I was right.
Jugg

GM used 5/16" for both Windshield, and Back Glass. Ford, and Chrysler used 3/8", and most imports of the era were 1/4"

5/16" usually works best for original thickness glass, which includes most replacement rear glass. However most replacement windshields are thinner and 3/8" works best on those. If the ribbon is too thick you have trouble getting the trim moldings back on.

Where can you pick up the 5/16 sealer, because I was looking in Original Parts Group(OPG) and all they carry is a 3/8 one. thanks

Year One has it. I'm guessing most body shop supply places have it too.