The technique I use involves a mortise and tenon joint.
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Most recently used on a truncated M-24/47 Mauser to install a 21" 7.62mm barrel while maintaining it's military appearance. I cut a couple of inches out of the fore stock and re spliced it under the middle band.
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The cleaning rod channel was reamed out for a couple of inches forward and aft of the joint, and a 1/8"NPT brass threaded nipple was inserted and glued in place to reenforce the joint.
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Since the barrel length was reduced, I had to drill the channel and re locate the cleaning rod stop further down until it butted up against the forward receiver post.
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Of course since your project is a restoration rather than a modification, that shouldn't be an issue.
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In the restoration of muskets I have also used a vertical slab splint set into a slot cut through the rammer channel or cut a mortise out of the aft channel and a tenon extending aft of the forestock ram rod channel, which conceals the joint if you fit the joint properly.
On this US M-1816, I made the tenon out of what was left of the sawed off original stock, and mortised the forestock replacement:
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Since the stock had been cut behind the band, I had to fit the new wood back to the lock plate base edge to conceal the joint as much as I could:
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If you are making the forestock from scratch, do your splicing while it's still in the square, and shape it after it has become one with the existing stock. It's a lot easier to manage & clamp that way.
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