Had the exact same problem not too long ago actually
Called up a friend with a 500. Said he just removed the stock first and I had to facepalm myself for not noticing. The internals are freaking amazingly easy to put in and out after the first time
.I wouldn;t cut anything or even try to sand it down. When my ATI Pro stock came in it stated that it shouldn't be pushed together. Leave a tiny bit room left together within the holding spaces. Guns are expensive, don't damage anything!