When I go over 65mph the front end "hops" like it's running on tires with flat spots. I've had 3 different setts of wheels/tires on it and it does the same thing. The front end was completely rebuilt and only has about 10k miles on it.

Possibly related; there is a clunk in the front end when the steering is turned to the left under acceleration from a stop. Like making a left turn from a stop sign.

Any ideas what it could be?

I'd get the car to a good frame and alignment shop ASAP and not drive it any faster than necessary. I am not trying to scare you but the car is telling you somethings wrong. I've had several cars over the years that came in with blatantly obvious problems that other shops had blown right by. Imagine a K frame rotted out to the point that just the top portion was holding it together. I saw it within 2 minutes if that. BTW, the car was just aligned for the 3rd time. keep us posted

The frame and k-frame are in great shape. I cleaned and painted them myself so I know they're solid. I've checked the torque on the k-frame mounting bolts and they're fine. I've also checked the torque on just about every bolt on the front end and they're all at the proper torque. The front end is solid.

Bad lower ball joint maybe? Do a complete front end inspection. One of your new parts was bad is my guess,, especially if it drove just fine for the 10,000 miles then all of a sudden starts hopping around at 65mph. I agree that it sounds like a tire flat spot OR a separating belt OR an unbalanced tire/wheel.....but if you've had 3 sets of known good new balanced tires/wheels on it, then it's the front end like you think. Maybe even some upper or lower control arm bushings? Just check. The way I used to do it is this
1. Lower ball joints - jack the front end up so tires are just off the ground. stick a strong mop handle or metal bar under the tire and pry upwards feeling for clunks
2. Upper ball joints - jacked up, push/pull really hard in/out on the top of the tire
3. Tie rods - jacked up. push/pull really hard on the left & right side of the tire
4. Bushings - just look. I dont' really have a good trick for that

My vote is blown tie rod end(s)

The problem with checking a lot of this stuff is you jack it up and let the suspension hang down and try to move it. Well, at that point all of the "stuff " is not at its wear point and may seem to be tight there, but not at ride height.

I was planning on doing a disk brake conversion anyway. Hopefully I'll find the problem when I swap out the spindles.

how does the car "steer" when it hops and makes the clunking sound ?

It's steers fine. Even when it's thumping at 70mph it goes straight. I can let go of the steering wheel and it drifts slowly right as you'd expect.

Those would likely cause a vibration in the steering wheel,
We need a better description to try and help.
When it's "HOPPING" have you tried the following:

1. Increase speed/ better or worse?
2. Putting it in Neutral and allow it to slow down through the area (MPH) of the problem?
3. Gentle Turn L or R (hard to do at 65MPH) Think 4 lane road lightly traveled
4. Harsh turn L or R under same conditions.
5. Try light steeping on the brake pedal. A: can yo feel the vibration? B: Does it go away.
6. What do you mean by HOPPING?? Body jumping up and down? seat of the pants feeling?

I think I felt what could be described as a hop when I had a bad wheel bearing up front. Felt only at certain speeds. Usually high speed or when making a hard turn, like a 90 degree turn as OP mentioned.

I just fixed a front end clunk that was trans cooler lines slapping the steering box mount area every so often if things were just right. Car was new to me and last guy gave up trying to find that. Once I felt the slap a few times I homed in on it. That won’t be your hop, but might be your clunk, something like that.

The tips posted for checking a front end are good. I’ll add that years ago I was able to find a bad tie rod or idler or pitman by reaching under the car not jacked up and pulling the steering linkage back and forth. At least I think I did that. Good luck. Let us know how the mystery turns out.

By "hop" I mean the steering wheel vibration feels up and down rather than side to side. It feels like driving on a tire with a flat spot but it doesn't go away.

If it's a bad wheel bearing it should be corrected when I do a disc brake conversion in the spring.

I'll take a look at the cooling lines.

Hi Duff , Did you replace the front strut and lower control arm bushings ? Some of the aftermarket strut bushings are made out of garbage rubber , and dimensionally are NOT made like the Original bushings allowing movement front to rear , especially with that metal sleeve that slips over the front portion of the strut . Some of those sleeves are 1/8" too long VS original , causing movement , check carefully . Ronnie

So am I to assume you're not currently driving the car? If you are and have a wheel bearing that bad it may damage the spindle as well or lead to other unexpected failures. So I would encourage you, if driving it, to pursue the cause

What is the frequency of the "hopping"? At wheel speed, a fraction of wheel speed?? And I'll repeat what TJP asked, once it starts, does the frequency change with speed or is it constant?

Uhh, not that this was same thing . . . but - on my 70 Sport Satellite, small block, I installed the power steering pump with everything that was with the car - BUT, it was missing a plate. That allowed the pump to vibrate - caused vibration in the steering wheel, but not the rest of car. Up to about 60mph, all was good, over that the steering wheel would really vibrate in your hand. Turns out that without the plate, the pump was vibrating, and had cracked the mount in such a way, it was keyed together, until you got up the speed/rpm . . .then it would come loose and vibrate !! . . . I went thru the usual, balance front wheels, exchange wheels front to back, check out front end . . . wasn't til I was looking at another small block with power steering that I noticed the missing plate !!! . . .

It's the triangular plate in upper left of pic (Thanks Hoffman's winners circle). . .

Must've been a fun one to find

It's currently in storage. Winters are brutal here (Wisconsin) and salted roads destroy old cars. My goal is to be able to take the car on a 200 mile trip in the fall, so I want to be able to start working on the problem when the car comes out in the spring.

I haven't driven it since October, so I'm not 100% sure but I think the frequency increased with speed. Maybe it's always there but the frequency isn't high enough to notice until about 70mph?

When warmth returns to the frozen north and you have a chance to get her back on the road, it sounds like more diagnostics are in order. Will be waiting with bated breath!