Couldn't tell from your pictures or not, but you could also put a stiffening plate in the frame where the bolts come through. Have the metal of the frame sandwiched between two thick pieces of steel instead of relying on the surface area provided by a washer. Assembly would also be easier if the stiffening plate had the bolts tack welded on so you'd just slide that in and wouldn't have to rely on holding the head of the bolt with a wrench.