Mounting Rod Holders w/ enclosed gunwhales?

[ElBichoMalo]

I need to install new rod holders (currently have none in). I can feel around in the hole, and it seems like some backer wood is missing, some is soft. I know the preferred method is to thru-bolt the rod holders, and at the very least to screw them into wood backing below.

Here is the hard part....I have a 73 SF20. The gunwhales or "inwhales" or whatever are fully closed to the deck. Is there some kind of trick to this? I would hate to have to add pie plates in the inwhales just for installation.