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Thanks for the help! Problem solved! On closer inspection, somebody had welded in a 1/4 inch plate up inside the cross member, probably to strengthen a crack repair on the front side of the cross member. A hole was drilled through this (much easier than cutting in a square!) which is where the round-head bolt came into the assembly.
So, I clamped the spring tightly in the cross member, extracted the square head bolt, then re-inserted it down through the top and tightened on the nut. The bolt sits down in its square home and all is well. I just have to make a note of this for future reference if and when the spring comes out again.
This isn't the first round headed bolt that I have found in a front spring and I wonder if I was seeing the result of repairs or did Ford actually use such a bolt in some cars?