I used the Herculiner in the quart size since I'm only doing the cab. Turned out pretty good. like to add another coat if it allows (didn't read that far) and use a roller this time. I think that would look better, but then it will be covered in carpet. Used it for leak prevention on the new panels over the seam sealer, and I'm sure it will quiet things down some. Post up some pics tonight.
I know you have to stir the crap out of it or all the rubber stays in the bottom, DO NOT SHAKE! Why? I don't know; but it says not to.
Oh and on another note; stay away from the Rustoleum spray, might be good if you can get it out of the CAN! Waste of money at almost $8.00 a can.