Rejoining old frames can be a pain, especially if they have warped over the years. On a bigger profile, sand down the miters to remove as much of the old glue as possible and flatten any divots where the old nails went in. The good ole Dremel is with a sanding drum is good for this, but avoid the edges. The miters are unlikely to go together perfectly, so there are going to be gaps. A slow-curing epoxy can work well as it will bridge gaps and is not as brittle as the instant sort. Drill a few shallow holes in each side of the mitre for the glue to squeeze into. Apply the epoxy liberally on all the mitre faces, working it into the holes, then put a strap clamp around and cinch it up. Leave overnight. While it is clamped, you can add the L plates to strengthen the joins. Before you fix the plates, place them c/sunk face down on a piece of wood and give them a swift tap on the corner with the ball end of a hammer. This bends them slightly inwards, so that when they are screwed to the frame they are biased towards pulling the faces of the mitre together. Bit like a v-nail. Speaking of v-nails, a lot of old frames, particulally ornate ones are on a carcass of really rough pine and don't take kindly to v-nailing. Also the clamp pressure can crush/loosen the gesso that will have shrunk over the years.
