I did it on our 77, there are several ways to do it, all depends on what route works best for you. its simple and straightforward. figure out how much you want them moved in, then get a good look at the brackets. I modified the originals, , others have done slightly different methods,...the end result is the same. see what will work best for you. I also sectioned the bumpers so the ends would also be in closer.here is a pic(if it shows up)http://mmb.maverick.to/showthread.php?t=11225
I have done this on my 73 with the frount bumper i can take some pics if you want I cut my mounts and moved them back 3.25 inches the bumper sticks out the exact same amount as the tip of the hood( no more bumping the plastic garbage can to know when im in the garage lol) I think it looks good
moving the bumper in I moved mine in by gutting all the internal bracing and shock absorbing crap and just using the chrome outer skin then I set it up where i liked it measured the distance from the frame rails to the bumper and built my own brackets to bolt to the original holes in the frame rails. ( MY CAR IS A STRIP ONLY CAR! I WOULD NOT RECOMEND GUTTING THE INTERNAL BRACING OUT FOR STREET APPLICATIONS! ) The chrome skin is very flimsy and would not help at all in a front impact. In case anybodys interested the weight savings was huge, the factory setup weighed 107lbs!, now my bumper with brackets weighs 37lbs!! Here is a few pics of how mine turned out , before and after. as for the plastic splash apron, I just cut it along the ridge in the molding and set the bumper up to match it. Now from the front it looks like how they should have made them!