Longbox55 is correct, 1/4 inch filler is too thick. I see no reason to cover the entire roof with filler. I used to be a bodyman & still am in & out of body shops every day. No one I know covers an entire panel because of expansion or any other reason.

1957 Chevrolet 1/2-ton Stepside LB in the Gallery
My Photobucket shots

The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." - John Stuart Mill (1806 - 1873)