The piece your referring to is a barrel block. These are used when very long and large barrels are used to support the weight of the heavy barrel. The barrel is screwed onto the action as normal. The barrel block is then epoxied onto the barrel and the barrel block itself is what is bolted to the stock. The action is free floating from the stock just like the rest of the barrel is. Perfectly stress free.
Since you have the wood already I would check with Joel Russo on a price. This stock had the curly maple already glued up by the customer and the customer supplied the birds eye but Joel glued the birds eye up and carved out the stock for $350. Here's a link to Joel's site