1973 Bumper mount: (image source: http://www.cpidshop.top/bumpers-c-132_158_1354_2265/73-1973-datsun-240z-front-bumper-bracket-pair-rh-lh-oem-used-p-3459.htm )
1973 Bumper filler (because they are wider)
vs 1970 bumper mount:
I switched on my 73 240z to the early bumpers and only bend/bored a self made early bumper style mount. For the reverse, you have to set them a bit forward but the sides will have big gaps without the bumper fillers and I hate them, I already destroyed 2 pairs because the material gets brittle over the years.
Or you would have to cut and weld the side mounts inside the bumper to a new position.
Here is the 73er bumper installed, you can see the bumper fillers behind the bumper and at the sides.