My experience was somewhat different. Now granted this was probably about ten years ago, but I bought used front and rear bumpers for about $300 for the set and had them re-chromed for another $300 - $400. The rubber on the front pieces was good, so I only had to replace the rear rubber which cost me around $100. I had the body shop put these on when the car was getting painted and repaired, so maybe this kept prices down, but they didn't even charge me extra for the work and the total charge for the paint and body work was $3500. They did an excellent job; you couldn't tell the euro bumpers were anything but a factory install. Again, this was ten years ago, but I just don't see why this is a $5 - $6K job. Mounting the rear bumper is about as straightforward as you can get and while there is some fabrication for the mounts in the front, I don't think they are making brackets for the space shuttle. The biggest cost nowadays, I imagine, would be the parts, but be patient and wait for someone who is parting his car out or given up on his project. Also, look for a smaller/less well known body shop as they may be more willing to tackle a conversion project like this for reasonable money; I visited at least a dozen body shops before I settled on one. I have since bought another '74 with the big bumpers. I plan on taking off the bumpers and putting a CSL (fiberglass) bumper for the rear and going bumperless with an Alpina air dam in the front. With the cost of the bumper, air dam, and body/paint work, I should complete the project for under $1400.
good luck,
Walt