The best blockoff plate would be to swap in an EFI non jet valve head, but absent that, all you need to do is pull the pump and push rod, fabricate a 1/4" steel plate that covers the hole, drill it for the mounting holes, cut a paper gasket and bolt it in place.

I have seen electric pumps used in combination with the mechanical pump, but I wouldn't even think about it. Usually, the mechanical pump is shot and doesn't deliver enough flow and the next thing that could happen is the mechanical pump diaphragm will rupture and the sump will fill with gasoline. Once that happens, you're going to catch fire and have a nice Montero bar-b-que.

Frank