High Speed Vibration 72 Duster help!!!

On a motorcycle, we'd call that a speed-wobble. And it's usually due to the front and rear wheels not on the same centerline, or the rear is delivering a thrust angle, that is; the rear end is not centered or not adjusted correctly.
So you have a 73 A-body front track, and a Challenger rear track. Question is are the rear wheels a different backspace, to put the track back to stock, but more importantly, did your alignment guy find a thrust angle component to the rear install.
If the rear is driving the chassis at an angle, then the front alignment may be wrong. We used to call this dog-tracking, or doing the Nova-crawl.At slow speeds this is not an issue. But as speed increases, the rear starts to push the front sideways. Well the tire sidewalls don't put up with that for very long,before they start to complain. Then they stretch and relax, stretch and relax, acting like springs.Of course this leads to a wobble as the steering system can't handle that either. This usually has a narrow window of occurrence, and if you blast through it, you may not encounter it again, until some later higher speed point.
So as a test, pump your tires up to the maximum listed on the sidewall, and retest. If the problem is eased or moved to a higher speed, then this is likely what's happening. You can also have someone follow you to observe for dog-tracking. Dog tracking, if serious, must be corrected. If minor, a thrust angle alignment may be able to compensate.
You should be able to see this as wear on the front tires. there will be a feathering of the individual treads on each tire, and both will have the feathering on the same edge of the tire; either the left sides or the right sides. This is a completely different wear pattern as a toe problem. And as you can imagine, it's a rare pattern.
Now having said that, it's also possible that the alignment slipped, although that is usually accompanied by a pull.
You need to get this figured out ASAP, cuz every mile those tires turn, may bring them closer to a non-recovery state. When the sidewalls flex like that, they build a lot of heat. Tires do not survive long under that condition, before they start to separate inside. Then they leak air...forever. So you tube them and put them on the back. Where only 70% sits flat on the pavement until they wear flat again; which might not take long ,cuz they are easier to spin.
And sometimes they cup. Once a tire is cupped, it may never recover.
Don't throw the front clip away, I seriously doubt it is the source of your woes. It takes about that 5 minutes for the tires to warm up, and soften up. Cold tires are stiff and much harder to flex.Lots of A-bodies dog-track, esp the lwb Darts.I really think you will find it in the thrust angle.