I occurred to me that, rather than butchering the butt, would it not be better to destroy the screw?
Run a series of drill-bits up the hole in the butt and steadily cut away the head of the screw. Once it snaps away, use the "usual methods" to "tap and wiggle" the wood to the rear.
Once that is off, choose your best method of removing the main body of the screw; hack-saws, Dremels, etc............ Obviously, with a SMLE type, with the square "extension", you are still going to have to remove the fore-end, but undamaged and under control. It the screw has been hacked off, and the fore-end removed, the remaining square stub can also be grasped with the "wrench" (Box, open-ended, Vise-grip, etc.), of your choice and then carefully wound out of the body.
Chase out the thread in the body. It is as close as you can get to a 7/16" 14tpi BSW, i.e. Whitworth standard, the ONLY such thing on a SMLE) and is EXACTLY the same thread as on a No4 / 5.
All you then need is a NEW stock-screw and a bit of grease.
Also bear in mind that SMLE wood was soaked for ages in linseed oiland that the tapered end of a "new" butt, was factory-fitted using what was essentially a powerful press. Once in place, the assembler whacked in the big bolt and single washer and wound it up tight, with the forward-protruding square stub being aligned using a gauge.
Nothing like "living history"!!