You could always use a 2 part epoxy to build up whatever surface needs a little thickness. This way, you can sand/file the epoxy buildup to whatever thickness is needed. Also, I've learned that it is is very helpful to mix in a filler such as fine sawdust into the epoxy mixture as this will help tremendously when you are sanding the epoxy since cured epoxy can be very hard stuff.
I dont know how bad it is but I'd just bed it to the reciever if it was mine & it bugged me. Coat the reciever front with release agent as well was around the front. Useing a toothpick or something similar place a small ammount of epoxy bedding (I use acra glass) material around the forend where it fits into the reciever and reassemble it. If you didn't use too much it wont ooze out but if it does just wipe it off while fresh. Once its cured your forend will be a perfect fit for the reciever.