A old Chevy truck history book says that the 53 had the spare tire mount as an option. It is on the driver side and required the fender indentation. Those boys in the 50's were pretty clever with body work. I imagine you have a customized fender, probably made with some roller tools much like the way they make
gas tanks for motorcycles . That is my guess.
Should be to hard to press it out using body tools or find a motorcycle custom shop and see what he recommends
