Here are some photos of the backing plates that I made for my Cabriolet. I threaded (no 12) pieces of iron and welded them on the backing plate
You assemble plate, bolts and nuts to the hinge, and bolt it all together. This will locate the nuts perfectly with respect to the countersunk in the hinges, while you weld it up. By the way, you should also put spacers of sheet metal of the same thickness as the lid panel between the backing plate and hinge while you are welding it up. If you do not simulate the thickness of the lid in this way it will end up wrong.
Also, just came to my mind -another argument for using the backing plate is that you can drill the holes in the lid inner panel slightly oversize, and thus have an additional possibility of adjustment when you mount the lid to the car.
Finally, you should be aware of the differences between original and repro hinges. There are several threads on that subject here, so do a search.