I always do the required sheetmetal to a car before I start the modification process when there is not major unibody frame work required for a future upgrade or performance install. I have found this makes the finished product better since I can "finish" the work to a higher standard before the modifications begin.

I think it is easier to install the sheetmetal and then modify the car/sheetmetal as needed for upgrades than installing the new replacement sheetmetal "around" a non-stock performance modification. Again, this is based upon the fact that you are not making your own frame on floor pans.



Besides, to build a car properly you actually build the whole car and then tear it apart for paint/body work/interior/re-assembly etc.