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The "stronger" design that Larry has mentioned was in fact released much later for service.
The "basic" pan for '39 and '40 were nearly identical. In '39 had there was a little "foot" near the front - bottom edge that was bolted to the top of the frame horn. This, along with the two short braces that went to the front cross member were the most obvious differences between '39 - '40.
In '40, the entire bracing design was redone. That's when that large "horse shoe" brace and spring were installed at the center of the cross member and was attached to each pan.
Some of the '40 (proper) pans still had two holes at the lower - front corner of each pan.
It appears that some of the dies simply had the punches removed while others did not.
I'm not certain if you would gain any significant strength by installing '40 (proper) pans vs. the '39 pans.
The cracks were typically induced by flexing (stress). Perhaps if you'd install the '40 fender brace and large spring at the center you may eliminate any future cracking.