I've done this type of replacement, and found that a small excess in the hole is not a problem. Proof - boat has not sunk yet in a few years. As noted, you can fill your excess hole size at the same time you mount the G10 backing plate with thickened epoxy and hold it in place with the through-hull. Rather than mixing it up yourself, why not use West Six-10 which is thickened correctly and is easy to dispense for a small job. I coated all through-hull parts that I might want to remove later with mold release wax, but I'm told that ordinary Carnuba car wax is the same stuff. The through-hull fitting was removable by unscrewing and I've got a nice set of threads in epoxy which are sealed to the hull, in addition to the seacock mount on the inside. The Six-10 filled the irregularities on the inside of the hull and squirted out the side, and I just used a gloved finger to fair it out in the corner, like caulking a seam. It's a permanent part of the boat now, so I'd rather not know if one of you experts says it's going to fail next spring!