Save yourself a whole lot of bother and get a No 42 or 53 with what looks like a horizontal AND vertical crosswire. These are actually etched onto a glass plate (which changes the nomenclature from a graticle to a reticle). You then have an already (reasonably) accurately collimated centre-dot/spot to aim with instead of the vertical line which was OK when using it to aim the BESA gun on the Mk7 Churchill but, alas, not a good way to hit targets with a rifle.
The BAD thing about trying to convert your existing No42 inner sleeve (the long brass bit that holds the wire) is that to fix a NEW wire and post, you have to heat up the whole thing to the melting point of solder before it will take. If you insist on converting the glass reticle to cross-wire and post, you can kick out the plate and fit the new wire and post. And you only have a simple 1/16" thick plate to heat up
PS, the reticle plate is held in with screws. Easy to undo, work on and replace.