A 6mm pivot pin with holes for retaining rings at each end, can be made up by local marine stainless steel suppliers.
The tuning guide settings for the spreaders are not an exact guide. Generally the guides have been written with wooden boats in mind, whereas GRP boats have slightly different shroud positions.
It is the rake and prebend measurements that are important, and the spreader length and angle can be adjusted from the starting point in the guide, to achieve these settings. Looking along the tensioned shroud, one can see which way it deflects at the spreader end, and so get an indication of what forces/prebend the shroud is applying to the mast. Raking the spreaders back will increase prebend.
After getting the prebend and rake correct by adjusting shroud length and the spreaders, also with the pivot pin loose, chocks are fitted to limit further lower mast bend such as when the kicker is tightened (not to create prebend). The chocks should only be used in front of the mast.
The mast track rear pin can then be positioned close behind the mast foot, and small metal shims added to fill any small gap remaining between the rear pin and the back of the mast foot, so that the mast will be located in a consistent position, with the pivot pin loose, each time rig tension is applied.