The 'best' location would be as close to the transom as possible, maybe within a foot or two, and a bit up from the keel. . . . However, the engines being in the way do not make that location easily accessible for a retrofit.
As an alternative, you could be about 3 feet from the transom and still have a decent location, and better access for installing the thru-hull pickup. Your key concern would be 'catching some air' in rougher sea/lake conditions.