highway vibration

i have a 72 duster that used to have a slant six and now has a 360. all of the major suspension parts came out of a 73 dodge 340 car in cluding the rear end, k frame, strut rods,transmission, and brakes. i have now completly replaced all of the front end peices and rear leaf spirngs and u joints. the new front end peices include, tie rod ends, brake calipers, roters, ball joints, swaybar links, idler arm, pitman arm, power steering box. i have unilug rims all around with brand new tires, the front end was allinged twice because the first people messed it up and the front tires were replaced each time. i have taken the wheels off multiple times to make sure they are centered. the problem im having is that after about 5 min of driving on the highway a horrible vibration starts and the steering wheel shakes and it feels like the wheels are comming off then when i brake to slow down or get off the highway the vibrationg gets worse and the vibration can be felt in the pedal but once i stop and start to drive on the normal road the vibration goes away then once i get back up to 40 the vibration comes back but before i got on the highway it was not vibrating at 45mph. i hope i gave enough info any help would be great. i have replaced every thing now.