Without figuring the shear strength of the bolts (after consideration for fatigue) vs. the backstay -- by inspection the cross sectional area isn't the issue. I think the issue is precluding movement in the chainplate.

The chainplate's controlling issues are going to be keeping the chainplate in place through the transom. That will depend on torque the bolts down to create friction, plus the shear capacity of the holes through the reinforced FRP.

The backing plate must marry the bolts more closely. From what Dan y says -- I wonder if the backing plate is properly sized for the 1/2" and the visiblle chain plate is just oversized. It's also possible that the backing plate doesn't exactly marry-up to the chainplate itself (i.e., the centerline of the holes doesn't "match-up")...

Dan y - if you had 1/2" bolts on there before, I'd make sure that you have the same grade when you replace them; also I'd take the old bolts down with me, just in case....