You need to measure the chokes before you do any mods. If the gun is a pretty rough field grade, refinishing it properly won't hurt its value. If it's a graded gun in relatively decent shape, it'll kill the value. Best to get some input from experienced sxs people first. Posting good detailed pix here would get you some good advice.

Restoration work is a multitude of skills. You should read the book at the top of this forum if you haven't done this to doubleguns.