I got some flat head carriage bolts, but the head was smaller than the plow bolt. I finally decided to modify an elevator bolt. With the bolt in the bench drill, I used a file to take the diameter down, and then a grinding stone in a dremel to profile the countersink. I could go a little tighter on the countersink edge, but I am satisfied how it turned out.
Watch out for careful drivers!!! I'm away on an ego trip. Will be back on Feb 30. I'm not an Auto Mechanic, but I play one on TV. I charge $0.02 for every opinion and I take Paypal. Plan B is always better than plan A, by definition. I used to think beer was bad for me, so I gave up thinking. Sometimes no nonsense makes sense, in a sense. You can't teach a new dog old tricks. Honk if you're Amish