You may want to inspect the area of the firewall around the
clutch and brake pedal mount.  That vintage with manual
trans had some failures where the firewall cracked and would
sound like a suspension thunk.  IIRC, there was a service
bulletin and a repair panel to address this problem.  If
that is the problem, the pedal mount will fall into the
engine compartment.

Another thing you may check is the crossmember rivits as
they would occasionall get loose.  The fix was to drill them
out and replace with really good bolts.  A few suffered
cracks near the steering gearbox in the left frame rail
which should also be checked closely.  I also saw a couple
of gearboxes get loose on the frame for some reason.

Now, you have a to-do list.  Let us know if you find
anything.  A noise like that needs to be identified before
you can know for sure that it is safe to drive on the same
roads with the rest of us.

Lugnut