The slight thickness you'll note with my stock roof '33 Willys coupe body castings now are from remolding the body 4 or 5 times using a resin copy for each 'new' mold.....since I made changes along the way to improve the accuracy of the body. It is now something that I pay real close attention to when I use a resin body to make a 'second issue' mold. You do not see the slight difference in thickness until the third or fourth run; or if you use a casting from the end of the run when the mold has gotten weaker.......allowing the resin to push the sides out a hair more. Since catching this problem I've made sure to thin the bodies down more then the original master so that they will be thinner yet as the life span of the mold declines.
On another note, Mill City Replicas' stock roof '33 Willys body is nice, but no where near as accurate as mine is.......if that is something that matters to you personally. I spent months making sure the proportions were as close to the 1:1 as possible since I dig '33 Willys so much! LOL! Also, the casting I had from them years ago was pretty thick, full of voids in areas such as the grille, and the front end and running boards were all molded to the body like a promo. I am not bashing them in anyway. The body was given to me by a friend; so I have no idea if it was a 'second' that someone got into his hands.