Metal 2 Metal is very good stuff for the application, but really, dont just let it sit overnight and get at forming, just let it set up and start working it. The longer you wait the harder it is to deal with.
We used to have a guy come in to patch our lead lined anodize tanks, the tiny little torches he kept in his pocket did such beautiful work. McMaster Carr still sells bar solder for 25 bucks a pound stick, that is still 60/40 so its easy enough to find and try.
Right now we are replacing both the quarter panel and roof panel on a chevelle. Vinyl top and a garage collapse. Get to the between seem from the AMD roof and Goodmark quarter and it looks like a 2" strip of metal welded in would be a better choice, still dont know which way we are going to tackle that. I know the lead stays more flexible, but metal 2 metal is so hard, I cant see it being better than welding the two together as hard as it gets.