Bob, you can get some background on the Greenfield clutch from this thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=21336&page=2

It is mounted on a square shaft: the center sliding part of the clutch (which drives the axle) can be pushed either right or left (forward or reverse) by the pedal mechanism to engage either the forward-spinning or rearward-spinning V belt pulley, but is fully floating unless pushed one way by a spring or the pedal mechanism. The pedal linkage differed greatly between models, but if the total clearance across the center of the clutch, inside the two large V pulleys on the outside, is not very close to the correct value, the clutch will not work properly. On some models it even seems to go overcenter and lock into forward or reverse drive. The only cure is to have the correct plate clearance, which is determined by the shim stack which sets the separation between the two large V belt pulleys, in conjunction with the thickness of the linings on the two sides of the center part of the clutch. As the linings wear down, the linkage runs out of efficient movement, and you have to either replace the linings, or reduce the shim stack, to bring the clearance back to what it has to be. That requires pulling out the complete main shaft (the one that carries both clutches and is square in its center section), dismantling it on the bench, then reassembling it with a reduced shim stack, which allows the large nuts on the two sides of the shaft to push the large V belt pulleys in closer to each other.

So far no one here has come up with a way to adjust the clearance except by changing the shim stack (which is what you are supposed to do) or using an undersized shim stack and then trying to freeze the clamping nuts by using Loctite. I'm not fond of the latter approach, but a member did suggest some success with that technique in another thread.