The quarter panel to door gap & rocker to door gap are the starting points for alignment of the sheet metal as the quarter and rocker are not adjustable. The gaps on the old cars varied a lot however the main thing you want to achieve is consistency. For example you don't want to have a tapered gap were the gap is wide on the top and tight on the bottom. A good starting point is to get a even gap on the door the rocker panel. These tend to run a little wide but not usually a problem, again look for consistency. If the door to rocker gap is tapered this will show up in the quarter to door and fender gap as the door is not level. In my opinion the quarter to door gap should be at least an 1/8 and no more than 1/4 inch. Rule of thumb for me is if it looks to tight or wide it probably is. Getting good gaps on a mustang is not easy and even harder if you are dealing with reproduction parts and replacement welded on panels that may not be correctly installed. The good thing there is a lot of adjustment on the old cars and with patience good gaps can be attained...Chris
