Do you recall what the panel gaps were between the rear of the door and what they interfaced with on the car? 2dr or 4dr? If the panel gaps are currently decent and all body lines match-up reasonably well, then you can move forward with the fenders, getting them placed to continue any body lines forward. One side then the other side.
With body fenders "roughed-in" then test-install the core support and make the radiator opening level and centered in the middle. With the rubbers installed, but no shims. Might need to attach it to the fender sides snugly with bolts to get the measurement you desire.
Then, with everything secured where they need to be, insert shims between the core support upper mount and the core support itself. Torque everything down and re-check.
When a body-on-frame car comes down the assy line, there is no front sheet metal on the assy, but the core support is already installed loosely. Then, from behind a curtain, a worker totes out a complete fender assy, placed it between the cowl and core support, installed about 6-7 bolts, using plastic alignment guides on the sheet metal, and then walks away. Faster than you can say "440 6-Pack"!
Once you get the alignment finished, then take each fender off and put all of the other things onto each one. Then reinstall them "as complete assemblies", making sure all alignments are "to specs".
A "bad thing" is that Chrysler was not known for even panel gaps, even on the same side, as was Ford back then. Certainly there were specs, but apparently they had a wide margin of error.
I should also suggest the floor the car is sitting on needs to be verifiable level, front to back, side to side, for any variations can end up in the panel gaps and other alignments.
Take your time, one step at a time, and please let us know how it all goes.
CBODY67