You might want to check and see if there is a different flexplate for internal vs external balance 340. If not you might end up with the wrong one. Cast crank 340 is external balance. I cant remember for sure but I do know the aftermarket B&M has a different part number for the external balance piece.
I dont run stock flexplates anymore. I have seen them break before. The solid B&M ones are bullet proof.
They repro the covers now and they are reasonably priced. Fill tubes are tough to find used, always seem to be missing. I have several trans that are missing the fill tubes.