[J Witt]

IMHO, the problem isn't the distortion of the metal locally around the bolts, but whether or not you start a tear in the body metal. The large washers (about 5 sq in in area), which are about twice the body metal thickness are supposed to aid in not allowing the bolts to pull out by tearing the sheet metal. The washers are intended to bend along with the body metal.

If you add another stiffener, I wouldn't go any thicker than 1/8 inch steel to allow some distortion to occur, and also bevel the edges and round the corners that face against the body metal to prevent localized tearing.

Keep in mind that the belt mount points will deform the sheet metal when loaded by high stress. This is OK, but it's very important not to allow the belt mounts to tear through the body metal, thus the need for radiused edges and rounded corners. Several of the references I looked at cited a maximum of 1/8 inch thickness for the reinforcement plates.

Simpson recommends using a design load of 2500 lbs for each anchor point, but they are talking about race cars and 6 point harness. The referenced impact force for airbag/seat belt combinations for DOT is (IIRC) is 35G, but this at the impact point and is somewhat reduced by the structure crumpling. This means the force on the mounting points for a lap belt, with a 180 lb body is 6300 lbs, or 3150 per mount point. Think of being able to pick up the car by the seat belt. Keep in mind, the car isn't usable anymore after this impact.

I may be rationalizing some here, but the Model A is not a survivable container for 35G impacts, but a decently mounted seat belt will help you a lot by keeping you in the car until the action stops. This is especially true because of the poor door latches the A has.

If you really want to help the collision safety, get rid of the windshield finger pulls and the wiper motor.

All of the above is my opinion and you must make your own choices.

John