The problem you're having with NOS guards not fitting on stock bumpers is not a new one. My experience was fitting front and rear 1966 guards on my stock 1966 bumpers. I knew both the bumpers and guards were original pieces. None of the guards were close to fitting. Here's what I found out from my chrome guy. If your stock bumpers have been repaired/straightened, they may not have been done to original contours. If original bumpers and/or guards have had high quality re-chroming (show chrome), the multiple layers of nickel will change thicknesses. This was my problem. The bumpers were show chromed and then I had the guards done, as well. I solved the problem by having my chrome guy carefully grind the back side of my newly chromed guards so the contours matched the bumpers. The whole project ended up taking a lot of time and money (original 66/67 guards are not inexpensive) and show chrome costs a fortune. In the end, the guards fit perfectly. There are also countless stories of fitness problems with repop bumpers and guards.
one more thing is both were made by a different company i'm betting an each had a plus/minus on finish profile so if one is to the neg an the other to the pos thats the problem. called tolerance stack up.