(3) 1/2" bolts through the stringer, with large flat washers, lock washers, and nuts. Probably don't need a plate there; there shouldn't be any inward force once the engine mounts are tightened to the brackets. All of the force should be down from gravity, plus whatever twisting torque the little 2.3 can muster, which should also be vertical forces at the mounts. When I redo the stringers, I'm contemplating fabbing up a nut-plate and glassing it in, which would eliminate the need for the boxes. Of course, an anti-corrosion sealer on the bolts so no water can get in once it's all together. Then, when I pour the foam, it should back right up to the back of the plate. If my plan is flawed, please let me know. I'd rather know now so I can plan the right method before I get there. As it is, I've been playing with parts more than anything lately. Every time I get some free time, an afternoon thunderstorm comes a-rollin' on in.