these cars are unibodies,they rely on all the panels for thier strength and rigidity,these panels are all welded/spotted together.
if you have put in tubes(cage,connectors,ect) to replace the strength then flexible sealer is more than good enough even with a small gap between the panels.if not it will flex depending on this or that,this may not be a big deal but it can wreck good body work and paint.
The car is a 68 Dart 440, 8 point cage with frame connectors, I will be running 13.5x28 m/t et streets, anyone who has built there own tubs will know how difficult it is to have "0" clearance between the tub and 1/4 panel. I built mine today, and have them sitting in place with 1/8 to 1/4 clearance. I am debating whether or not to remove the factory under coating on the inside of the 1/4,s to bond the tubs to the 1/4, panels? thanks for the help.
when I did mine I looked around and found some instructions on the web and they stated to have between 1/8 to a 1/4 in clearance between the tub and quarter. if you weld them to the quarter with 0 clearance the spot welds will pull on the quarter and show through making the quarter wavy