To get the mast to sit back in the track, put something soft (cloth, sponge etc) in the gap in front at deck level, reach up high above your head and shove the mast forward. The pin is partly there to “lock in” the mast’s correctly tuned position, once you have found it.
The whole tuning procedure is iterative, i.e. you gradually home in on the right setup, sometimes repeating a step at least once or twice. This flowchart (linked) may help to understand that aspect of the process.
When you can’t get the pin into the track where you want it, use spacers between the pin and mast foot.
The shrouds are not all that far aft of the mast, so the little pin also acts as a backstop to rule out the slight possibility of the mast slipping back.
Remember to measure the rake with the rig tensioned (mid-setting). The shroud length is what dictates the rake, and the pivot pin position is what diagnoses the correctness (or otherwise) of your backstop pin position. That’s the tuning process in a nutshell.
The Highfield lever offers coarse adjustments only – for finer tweaking you would need a muscle box for example (or a screwdriver and tension gauge…)
An 8mm pin should be fine, but 6mm diameter 316 stainless round bar is readily available online.