Cheers for that, Jim.
I'm now looking at making a custom piston using a threaded 77 latch rod, by doing this I can have the minimum wall thickness between the front of the spring and the rear of the seal, giving the maximum Spring room.
I've found some Titan springs in my pile with a 19mm OD made from 3mm wire which I think will fit the bill, by using one of these I can increase the wall thickness of the piston to bring the weight back up to make up for the lighter front end.
If I can manage to keep a decent wall thickness at the rear of the piston then I can fit a bearing to it to make up for me using mild steel for the piston body.
All the best Mick