I'd like to know what the actual weight difference is between the '76 steel and the '77 aluminum. I think the '77-'78 bumpers still had the steel back up piece, and the aluminum was just the shiny face. In that case, the weight advantage of the aluminum bumper might be minimized. If the weight savings were significant, I'd be hunting for the aluminum pieces. I did do a short search for them and found nothing in NOS condition on ebay or anywhere on the 'net for that matter.
When I was dismantling the car that has become the Cosworth Pinto, I removed the front and rear bumpers and replaced them with custom parts a friend made for me in his shop. The weight savings was phenomenal!
Funny story: I was looking around town for the Pinto for my Cosworth Pinto project, and I had spotted a car in a modest part of town that fit the bill. It was powder blue, a '74-'76, and in great, still-driven condition. I wanted to stop by and make an offer for the car, but had not yet done so when I found the car I eventually used ('74 trunk model, 2.0 engine, 4speed). I went about taking my car apart, removing those heavy bumpers, etc. Then I remembered the little blue car had a damaged rear bumper. It looked like it had caught on a post or something on the driver's side, and it was peeled back a bit. I had the bumpers in my truck to take to the wrecking yard, and thought I'd go by and offer to replace that damaged bumper with mine, which was in good shape. I didn't know the owner, but what the heck, I just loaded up the tools I would need and stopped by. The owner was a little older lady. When she came to the door, I told her that I'd like to replace her bumper for free. She said ok, so I set to work. Of course, the fasteners were rusted tight. I had brought some penetrating oil and a breaker bar, luckily. I crawled around under her car for about an hour, got filthy, finished the job, loaded her damaged bumper in my truck, and let her know I was done. I guess she had called her daughter(?) to tell her there was this crazy man in her driveway monkeying with her car, because a bit after I'd started, this woman showed up and went inside. I noticed they were peeking through a window watching me! Who is that guy? Why? Anyway, I told them goodbye and was on my way. I still checked out the car occasionally, but never approached her to offer to buy it, since I now had my own. About a year later, the car disappeared. Now I wish I'd have tried to buy it anyway.