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The original Ford stuff is usually pretty darn stout, probably SAE grade 5-8.
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Um. Ford bolting as found is slightly less than Grade 5. Grade 8 bolts are head and shoulders above much else.
Ford used quality materials. Vanadium Steel is common but not universal. The use of "rolled threads" on most fasteners augments what is already there in material quality.
You can probably get away with this, but go to the Grade 8. And be sure to use hardened washers perhaps on BOTH sides. You'll probably have more problem finding the hardened washers than the actual Grade 8 bolts.
Look to available online torque charts for torque requirements for Gr 8.
I went through this conversion of '40 type backing plates - and then backed away when I came to the (correct) thought that the limitation in braking is NOT the mechanical aspect but rather the square inches of rubber on the road and the less effective "presentation" that skinny tires affords. With mechanical brakes properly set up and tuned, hydraulics don't improve stopping - the limitation is in the rubber. And hydraulic conversion might even degrade it since the car "weighting" is different than a later Ford. (2/3rds of braking occurs on the FRONT - a ratio the hydraulic brakes are engineered to in diameter of piston - but the pistons are sized and more importantly "ratio'ed" for the later Ford - not the Model A - once a wheel skids, even one, braking for that wheel is ended.)
In conversion I don't remember exactly the backing plate bolt sizing but 7/16 seems more what I remember. I may have had the earliest hydraulic backing plates - No ratchets/auto-set.
Joe K