Yes, it is just one step at a time then take another step. I put together and took apart the back half of the body so many times, I lost count. Each time I got the fit a bit better and the assembly more complete. The bottom back corners of the quarter panels were a bear to get a good even fit to the rear cross sill due to previous rust out and old, poor fitting patch panel work.
left rear quarter fit before
almost good
right side, note terrible fender fit
better fender fit, still no work on right rear quarter fit then
better fit on quarter bottom. Boy it still looked ugly at that point.
...and with each fit change it is another assembly step, take apart, correct it, put it together and test the fit, do it over again as needed to get the fit even better.
Even though the body is shimmed pretty well now, the bottom of the cowl still needs many hours of work to make the bottom reveal shape fit and match the piece below the doors. Again some old patch panel work that has to be redone. No one makes a proper panel for Briggs Bodies that I know of. So I will be pulling it off again soon, correcting the fit to the best of my ability, reassembling the body, mounting it with the needed shims, rechecking the fits and hopefully be done until I pull it off of the frame for paint, next year.