High Speed Vibration 72 Duster help!!!

I have had this car for a while now thought i fixed the problem multiple times but today it just did it again.
History
- started as slant-6 car (no problems except engine blew up)
- got a donor car for suspension, 1973 dodge dart 340
- installed the 8 1/4 rearend and small block front end and replaced all ball joints and tie rod ends and what i thought was all of the rubber up front along with a used steering gear- all new tires and uni-lug rims
-car drove fine a few times on the highway then one day started the vibration i have not been able to get rid of since.
- had to get new front tires due to poor front alignment eating them
-replaced rearend with a 8 3/4 out of a challenger due to the bearings eating the axles on the 8 1/4
-vibration still there so had two different people help me change the front bearings at different times and several times re adjusted them.
- tried someone else's wheels on my car with no change
-found out that i did not change the front lower bushings and thought i finally found my problem
-replaced engine mounts with new ones as well as a new steering gear due to a leak in the used one
- got all new wheels and tires and had the car aligned by a old school guy that worked on these cars when they were new
-i thought the last three things fixed it until i drove it on the high way while it was not as bad its still bad enough i had to get off the highway

Now for a better description of the vibration it comes and goes as it wants but is only triggered by highway speeds and normally only starts after being on the highway for 5 mins. when it starts it starts in the steering wheel and then moves into the front of the car until its shacking so bad it feels like the front of the car is going to fly apart. But i can pull over on the high way stop then wait a bit get back on then the vibration is gone then i go for a bit more and it comes back. Today i got of the highway the vibration went away and the car drove 20 more miles to work at around 45 with no vibrations.

Sorry for the long post but i have not been able to figure this out for several years now and im about to the point of throwing the entire front suspension away and starting over.

Any help would be great.