I don't see why it couldn't be applied over POR15, but typically it's applied over bare metal that has a fairly rough (40 grit) finish for good adhesion. It can be applied over the factory lead work, although again usually I would have a 40 grit finish to go over with the All-metal. With lead I'd probably hit it with a body file instead though, as that isn't something you want to sand/breathe. I have used it over lead, typically what I do is knock the lead down a little bit with a body file and then go over it with All-Metal or the Evercoat version, Metal-2-Metal. That way the lead is low enough that I won't be sanding into it later.
If you have a crack though you'll need to address the crack before you do anything. All-Metal and Metal-2-Metal are just body fillers that are based on aluminum powder, they're not structural in any way, shape or form. Neither is lead. If you have a crack in the cowl structure that will have to be welded up before any of the fillers are applied. If the crack is in the lead, it will have to be removed.
