I know that feeling! My dad has a ‘69 Mach 1 (428) and we encountered the same problem at about the same speed one day on a narrow two lane road (Old HWY 61 in Tunica County Mississippi). The front end lifted and it put us in the oncoming lane...thankfully it didn’t send us to the right in the oncoming ditch!
Dad has since had the front end rebuilt and it is quite a bit more steady at those speeds. Must be the way the aerodynamics of those cars are as the air tends to lift the front end. My 1975 Ranchero was smooth as silk at 125 (400 ci motor).
Speaking of Ford front ends..have you heard of the Ford Death Rattle? I believe that is what it’s called. You can look it up on youtube. It is a problem with the F-250 and up Ford trucks where when you hit a bump at speed the front end shakes violently for a while before smooting out. I just came back down I-40 from a job and a guy in an F-250 hit a bridge expansion joint at 75mph and it was a sight to see...I had never seen this in person before. He had to slow down and drive down the shoulder till it smoothed back out.
Not just Fords. It affects any solid front axle, quite a bit more likely with coil springs than leafs, as the leafs tend to hold the side-to-side motion in check better.
It can be something as mundane as worn out tires (my issue on my '98 Jeep Cherokee XJ), or any of a combination of things. Chrysler just came out and addressed it in fairly current models of the Wrangler.
My wife and I witnessed it on a newer Wrangler as we were travelling through Salt Lake City a few weeks back. My wife saw it first. I told her that was what my Cherokee was doing before I fixed it. It's violent, and if you aren't prepared for it, can cause a bad accident. Even if you are prepared for it, you can't stop it, you're only along for the ride. It's a shorts-changer, for certain.
Not so sure, the '67 Galaxie would snuggle down to the road over 120...sure 'floated' before that, from about 90 to 115, but afterwards, there was some sort of ground effect that kept it down.