1st new car I ever ordered was a 84 mustang GT black on black T tops, 1st rain storm filled the dome light till water was pouring into the interior till soaked, shorted out all the interior lights. Took it back to the dealer 5 times and they never got it sealed right, that was just one of the problems with that F ing ford, last one I ever owned.These cars flexed like crazy. All that torque has to get dispersed.
In the 80's when the IROC craze hit and T-Tops were the big thing, if you looked, most of those cars only had 305's. They werent available with T-tops, 350's, and manual transmissions, unless you special ordered and signed a waiver.
My 79 Trans am with the olds 403, had a crease on the roof, between the Ttops.
What you have there is simply a stress crack. Unavoidable unless you use flex additive. This is why it costs $20k to paint a vette.
Also, on factory original cars those joints are clearly visible, not hidden. They look like a 'trough" that's been filled with just barely enough sealer to prevent leaks.I will suggest a seam sealer but I will also let him know I want picture of the repair .