A pump

Which 426 ??       D-21/210/220 series of engines will need the engine oil cooler removed. This will require removing one pipe plug in the block, behind the oil cooler and once that particular pipe plug is removed, pull out the oil blocker slug, so oil can pass vertically from the oil pump to the main oil galley. Failure to do this and you will have terribly low oil pressure. You'll also then need to continue coolant flow like it was originally OR replace the water pump volute with one from a non-turbo engine. Failure to do this will cause you coolant flow issues (lack of flow). Now you can figure out how to mount the pump with probably 3 bolts instead of 4, grinding on the pump housing, opening the hole in the front engine plate OR machine the nose of the inj pump to fit the hole. Always make a bracket on the tail of the pump to secure it from moving around.  Mark 2 block ??  I think make a plate to cover the oil passages on the side of the block, drill two holes and remote mount oil filtering of some kind. Coolant flow same problem as the old block..fix it.

I agree. But some organizations specify model "A" pumps only.