After much discussion and planning, here's what my Gunsmith told me he'd do:
Carefully measure twice, maybe 3 times. Hacksaw the stock to the final length I want (accounting for the Pachmayer Decelerator pad I'll have installed).
Cut, carve and shape a piece of walnut to fit fairly tightly inside the cavity of the synthetic buttstock. Glue walnut into the cavity.
Grind and fit the recoil pad to the new buttstock length, drill the walnut insert for the recoil pad screws, fit and finish.
Hope that helps.
T.