Backfilling an oversized barrel channel isn't a big deal at all. Just put a couple of layers of duct tape running the full length of the barrel on the barrel.
Use masking tape along the edges (running full length) of the barrel channel, fill it with your favorite glass bedding compound or epoxy and set the barreled action in.
Let it set till it's not tacky and trim off the excess with a razor.
Then pull it apart after it's fully set, remove the tape, remount it, and bingo you have a perfectly floated barrel all the way to the action.
If you use a good expoxy you'll also add a considerable amount of rigidity to the stock in doing so.