auto box problem

[larryd]

1968 420 G.
Problem now that I have got the tickover set at 500 and its ok I can ask this as I thought this was due to a tickover at over 600 revs.

When I select reverse I get a horrible clonk from the rear of the car.
The diff and drive shaft are ok they have been checked out. This happens to a smaller extent when selcting drive but mainley in reverse.
Its a lot worse when the gearbox is hot anyone have any suggestions?
Fluid level is good and clean.
One more question does this box have a kick down under accelaration if so mine does not work any Ideas or thoughts are welcome. The gearbox has no other problems and drives well
Regards
Larry D