Ah, Evert.
Embarrassingly, and weirdly, after a quick measure I only seem to have a 66.5mm stroke.
--- I thought I was over 71mm.
I'll double check in the morning and strip the action down again but it looks like I'll have to machine up a custom piston.
Alternatively, perhaps I could use a 99 piston and just replace the body with a 25mm one, depending on the 99 piston's length.
All the best Mick