Turning left or right at slow speeds results in metal-on-metal hard clunk underneath. I feel it under the driver's seat on the left side, but we can't pin down the source. We've looked at ball joints, tie rod assembly, etc. I don't have sway bars on this 1967 F-100 step side. Shocks are good; maybe the coil spring is popping?

 

Radius arm bushings?

 

How about your wheel bearings and lug nuts? Double check your king pins.

 

A really great test, which can show you virtually anything that's working at making noises in the front, is called by many names. But it involves having a helper turn the steering wheel back and forth (a half-turn in each direction is usually enough) while you lay underneath and watch what's going on.
Called the "dry steering test" or "the test" or one or two other names, the full weight of the vehicle should be on the ground so that the torque of the steering system can work against the weight of the vehicle on the ground. Don't lift it up at all.
You can see anything from a crack in the frame open up behind a steering box, to a tie-rod or draglink end that's looser than you thought, or trackbar bushings and mounts that are loose or worn, to even seeing things going wonky with the rear springs.
But in your case, since you are feeling it under the floor of the seat area, pay particular attention to the radius arm bushings, as 52Merc said. But don't ignore the mounts themselves either. A weld or bolt or rivet might be coming loose?

Any modifications done to the truck previously, or recently? Lifted or lowered perhaps? How long ago were the wheel bearings serviced?
Speaking of wheels and bearings, performing the test above will let you see loose bearings as well, or even a cracked wheel flexing.
It's often a real eye opener!

Paul

 

Make sure your steering box bolts are tight to the frame. Could also be a front cab mount.

 

That gets my vote.