Use you're head, there is never a good place to re-weld a frame. They crack or snap because of high stress in that area, it's not likely to go away is it?
That'll be plenty strong enough if it's welded well. Personally I'd get hold of some U6 or whatever grade the frame is, and make two little oval things to slot into the holes, with chamfered edges, then fill the chamfer with weld.
If you put a plate over the top, then all the stress will just move along to the back end of the plate IMO, and it might just go there.