I am thinking of putting in an in-tank electric pump when I swap out my fuel tanks ... Should I just leave the old mechanical bolted in or is there a blockoff plate I can use?

 

Why on earth would you want an in-tank pump? Put one on the frame rail so that if it fails it is easily replaced. You need one putting out around 5psi pressure. And you do need to remove the stock pump and blank off the opening.
Do a search here, plenty of info. Search for holley pump.

 

Good thinking about the frame rail ... I can put a filter, pressure reg and pressure gauge right there with it ...