I have an older double with a large cavity in the stock rather than a drilled hole. At the butt end there is 1.2" of wood and you probably lose 0.2" of it by the time you get to the front of the comb. That seem like very little to work with to me.

Steve ,
Check out my post on drilling a stock bolt hole !
Bill gave sound advice by suggesting , drawing a full size sketch of the stock and the bolt hole as accurately as possible in a plan view .
This will give you an idea of what wood you have to play with. If it is necessary and indeed possible , you can always glue in additional wood into the upper surface of the bolt hole.

If the stock was hollowed out to that degree it was likely for balance. Lay out your cut for your comb without regard to the hollow. Cutting it too thin from top to bottom could likely make it uncomfortable to shoot. Glue blocks in the needed positions inside the stock body and the comb and then inlet your hardware. It is easier to inlet the round hardware. remember to check what adjustments are possible with the hardware. most only move in 2 axis. if you need to move the back of the comb up you have to drill the holes for the comb to achieve that angle. Hope this has been helpful.