"Can I use the later P7509 by using the later model regulator with the triangular plug?" Yes, if you rewire the charging circuit to support the later model regulator.
True I forgot to add. Even so the voltage to the regulator must not have any "drop" or it will also overcharge
Yet another way to check that is to run a no10 jumper from engine block to VR flange, and disconnect the IGN terminal and run a jumper direct from battery, see if the charge voltage returns to 14