!971 GMC inner fender sealing

[prateroad]

I'm onto another truck today. I've never pulled the inner fenders off a fleetside but bought a bed without them. What does everyone use for a sealer between the inner fenders and the bed? I have both brush on and a caulk tube of 3M seam sealer. I have used the caulk 3M on the seam at the back of the cab on top but it seemed to crack. I don't know what the factory used there but I don't want any cracking. This is a nice bed.