So, turns out there is a way to access the underside of the gunwales (although I may have to remove wiring conduit to get good access on one side), but there is 4 inches of styrofoam/flotation stuff immediately underneath the gunwales that I'll have to remove to put in a backing plate.

So, assuming I do that (or pay someone else to do that), what are peoples opinions on how far fore/aft to mount downriggers? Most of the boats I've been on have them mounted fairly close to the stern, but there are already holes drilled on one side (didn't notice that before, and not certain if they are the right configuration) that I could reuse. They are a bit closer to the console than to the transom, which is further forward than I would have expected.