Took one of the horns off the '84 (it was sounding pretty anemic) and put it on my HF scale, along with a horn off the '69. The '69 horn is 18.65 oz and the '84 horn is 14.85 oz. Pretty close to a quarter pound difference.
The remaining '84 horn seems loud enough by itself so I'll just run a single horn and do without the weight of the sick horn. Not a big weight reduction obviously, but it's off the extreme front of the car (right behind the bumper), and the reduction is free.
Next time I'm at the junkyard I'll try to remember to take my portable scale along and check out a few of the newer cars. WTH, I need to get a horn for my streetrod truck too, so I'll see if I can find a lightweight pair.