If you are going to remove the stock bumper guards, you should also move the bumpers in to the 70-72 position and remove the rubber filler strips. The bumpers will look goofy if you leave them in the 73-74 position without the big bumper guards. If you move them in, you can also get rid of the heavy reinforcements behind the bumpers.
The front can be moved in by just drilling some holes in the stock bumper brackets. There are some square holes directly in front of the stock mounting holes. Drill those to 5/8" and they become your new mounting holes.
As for the rear, a 74 uses bumper brackets that look similar to the 70-72 pieces(flat bar stock) but locate the bumper farther away from the body. You'll have to cut/lap/weld the stock brackets to move the rear bumper in.
To figure out the differences between 73 and 74, you need to start by measuring all the holes in your stock bumper and posting that information online.(with pictures) Then, someone with a 73 can do the same. You might be able to get away with using a 73 bumper. If there are any extra holes, you can just fill them with an extra fastener.
If I had access to my 73, I'd measure them for you. All I can do is post this picture which may or may not help since it isn't a "straight-on" shot. From this angle though, it does look like the outside holes on the 74 bumper might be farther apart.