One of the best products by far, in my experience, for your application is Marine Tex. It comes in white and gray colours.
The gray is almost black when it cures, and is very strong. While it appears very stiff before mixing, it become quite fluid when the two parts are mixed together. You can get it at most marine supply stores.
I have used it many times for bedding in both synthetic and wood stocks, bedding scope bases like you are wanting to do, and recently used it again to repair a real mess on a receiver bridge. It can actually be drilled and tapped when cured, and is very resistant to solvents, even gas and diesel.
Ted