Welcome, jb. I understand that you said you want to remove the buttplate and add a recoil pad, but there is another option, less trouble and less costly. Unless you really want a recoil pad in place of the buttplate you can order one of the leather lace-on (or velcro) pads and have it on in a matter of minutes. They will add 1/2" to the pull, and I have added spacers inside the pad to increase that a little if needed. I shoot them on almost all my vintage doubles, even some modern doubles. Here's a link to them and a pic of one of my guns wearing one. They do no damage to the wood whatsoever, and do an awesome job with recoil, if that is important to you. Just a thought for you to consider.
http://www.kick-killer-recoil-pads.com 
SRH
