I wouldn't want the rebounds to be too 'slack'- it's not proper engineering is it?
The suspension movement is limited anyway, so I guess that the stub separates from the rebound spring on full bump, but not by much. Mulfab bearings use shorter mains and there is a comparison picture on my kingpin thread. The rebounds are compressed when fitted, I would say maybe a quarter/half inch. I know that because my leather gaiters which were cut to fit unfitted rebounds had to be trimmed to fit when fitted. My original rebounds were slightly shorter than the replacements which I put down to 'wear'. The obvious solution to de-loading the rebounds is to make up some spacers to fit between the lower tube bracket and the bottom plate. You can nip up the bottom plate until the rebound is just compressing then measure the required thickness. You may need slightly longer nuts & bolts, and you would have to put spacer washers between the kingpin and bottom plate. The limit of the spacer depth will be the safe useable length of the threaded part of the bottom of the kingpin. Alternatively with the right measurement you could space the top of the kingpin from the upper cross tube as there's quite a bit of thread on the top bolt.
