I need a bolt size

The AMK catalog is saying B and E bodies used 1/2-13, and my A uses 1/2-13 also, so I'd say try any 1/2-13 bolt and see if it works in your car thread-wise.

IIRC the A body needs a longer bolt than B and E because both the inboard and outboard sides of the box frame are threaded. If both are threaded on your car, check to see if the grip is slightly under 2-3/4. If so and the 1/2-13 is correct, then you need four grade 5.1 (or grade 5 will do) hex head machine screws with conical washers x 2-3/4. Use automotive grade 5 or 5.1, not hardware store grade 2. Length 2-1/2 might work if the grip including the washer is a few threads less than 2-1/2.

Thanks man. Top notch info.

If your 1/2-13 female threads in the frame of your car are dirty, rusty, or damaged by weld spatter or whatever, you can file a vee in the first few threads of your bolts so that they are cutting like a tap. Make the side of the vee about 90 degrees to tangent in the clockwise turning direction for best cutting. Since the bolt is not nearly as hard as a tap, it won't cut as well as a tap, but usually well enough to get by. The original bolts had "paint cutter lead points" which do the same thing, as shown in the AMK catalog photos of the B and E body bumper arm bolts.