Frustrated with inability to do body work!

Ok doors are on and Im happy with them. Pics posted in members restoration "66 Dart" Next are the fenders. My first question is when bolting the fenders to the car, can I use the foam sealer that is used for putting toppers on a truck instead of using a seam sealer between the flanges? That way if I ever have to take it apart again they arent glued together. Also does everyone use the factory bolts again to go back together or are you using shiney stainless or plated bolts and washers. I know its probably preference but thought I would ask. I haven't decided yet.
Upcoming body work that I need guidance on, The lower factory body line going up to rear lower door as seen in pic below is not on the replacement panels so I need to make them go away. What is going to be the best method? I thought short strand fiber glass or am I going to have to weld it then fill some?
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And then theres going to be the rear quarters and how to make the straight body line and keep the curve above the body line where their supposed to be. Heres my starting place below.
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Appreciate any help you can throw my way.
As always comments and suggestions welcome. You guys are great!
Rod