Thanks Brian, this would make a good new thread. "how do you like to join your panels".
Let me try to explain why I personally like the gap.
First, if you don't have a gap the panels will overlap with even the least amount of shrinkage. You can planish the spot weld and use the gap as a guide.
Secondly, it requires less welding heat to bridge that gap and still get off quickly making it easier for me to control the heat into the panel.
Thirdly, If you have the entire panel joined by nothing but small spot welds and can keep it planished as you go those last spot welds will not hurt you as much. The most shrinkage, as you know, always comes at the end.
When I chopped my first car I did it all with an acetylene torch and hammer welded it. it was a 49 merc. Lots of welds. Lots of hammer and dolly. The MIG welder is the quickest and easiest way to weld but the main draw back, as you know, is the welds are very hard and difficult to planish. No red hot weld waiting to be worked. That makes controlling the shrinkage throughout the process even more critical.