In case its useful, note that the gauge sender thread is 1/2-20 UNF tapered, and the idiot light switch is 1/2-18 unc(?) tapered. If you could find a barb or tail with the relevant thread it would be easy (and tidy) to pump oil in with a piece of hose.
Are you sure that the two ports feature different threads? The reason I ask is my Rover's original engine had the sender and switch swapped over when they left the factory. They would not screw in if the ports had different threads.