Some great ideas there guys. Here is what I was thinking since the FG fenders may not be pre drilled.

1- remove all the mounting boltts from the '53 bed sides
2- mount the rear fenders to the running boards
3- use a jack or assistant to lift the rear portion on the new fender to the appropriate hieght.
4- check the alignment of the existing holes and see if they are spaced appropriately to allow the holes to be reused to mount the FG fenders
5- fill the holes that won't work
6- mark the outline for the appropriate fender location
7- drill out the mount holes in the FG mounting flange
8- mark the bolt hole locations on the bed
9- drill out the holes in the bed sides and check final fitment of of the rear fenders
10- drown my sorrows or celebrate, either way with frosties

Thanks for the bolt removal tip Roger.