Have a dead stock 74 Roadrunner 360 4 speed. Between about 70-75/80mph there is a vibration. It is even more pronounced when I let off the throttle after a quick dash to freeway speeds.

This car has never been modified. It only has 22,000 miles on it. It has always had this vibration even after replacing the clutch, flywheel, new fan clutch, and then having the original driveshaft checked and rebalanced. If I cruise the freeway at 60-65 the car cruises nice and smoth and quiet. If I crank it up above 80 same thing but in that one area it sounds terrible.

Any thoughts on things to check? Thanks

have you thought about the rear end?
I had a Newport station wagon that would do the same thing.
turned out it was the bearings.

also double check all wheels for balanced with front end alignment -

tires could be a problem for un eveness/out of round?

Just a thought here, I have seen wrong U joints put in before, caused the drive shaft to slide up and down inside the joints??
jmo

You might also check the driveshaft-to-pinion angle.
Incorrect angle can give you vibrations.

my 19K mile hemi car did the same thing..new slip yoke and cured!