I wrote an article about L42 fore-ends for this forum a year or so back and wrote a little cheap book too. Have a browse through both, chat to thunderbox and come back if you're hell bent on converting what you have. Better to start afresh with a bog standard No4 Mk1 type fore-end. At least that way, when you router the barrel groove you won't end up with a heap of pure butchered tat (see article.....) that you can snap with a finger