Vibrations at speed

Hi All,

I have a 1974 dart swinger. I tore this car down to nothing rotisserie sandblast, the works.

Engine is a 318, presumably stock, bought it locally on a gamble, have finally managed to make it run smooth and seemingly well.

904 transmission, and 81/4 rear end literally farmers field parts car parts that I have drained, cleaned and put new fluid, trans filter kit, input/output seals into.

New rear leaf springs, manufactured using my stockers as template
4 New cheapo OEM style blue shock absorbers
4 new tires, balanced and correctly inflated
All new front end parts including pitman arm, Idler arm, upper and lower bushings, offset moog bushings for UCA
1.03 torsion bars and ride height and alignment done locally ( very little faith in local shop, they measured height using sheet metal.)
All brand new brakes and brake components including rotors and drums.
I have checked and rechecked bolt torque on everything and anything.

Now the question.

Car runs and drives very well around town and has as much power as a 318 stocker should have.

Get it on the highway and hit 77MPH ( according to my needle) and she starts to shake and vibrate pretty good.

I do not feel it in the steering at all, and it seems to handle well.

My question is.... how do I even begin to investigate this? What should I look for? Any common issue that may cause this?
My fear is that when I turned this car into a skeleton, I DID NOT brace the car, when I was welding new steel onto her, it was pretty obviously not 100% square and symmetrical.

any thoughts would be welcome
thank you all