It's tough to get in behind the panels to repair from behind, and some 'Vettes will have a backing strip behind that seam that will keep you from getting at it as well.
I always just grind it out from the outside. 1.5" on either side of the seam should be lots, just remember that the seam goes the entire length of the panel even if it hasn't cracked YET. Grind it out in a V shape and lay a few strips of fiberglass matt in there to build it back up to level. Sand, fill and prep for re-paint. Don't use woven fiberglass mat, use the random-direction stuff and a good quality resin from a body shop supply store.
Once you're done, it should be structurally one piece and your cracks won't come back. Wear a dust mask when you grind!