The Empire 48 inch straight edge / ruler (model 4004 M) gave me just what I needed to measure and flatten the top. I checked the straightedge on a "reference" table saw, and it is perfectly straight.
I used the new straightedge to measure my saw table excluding the wings. It was flat in every direction except diagonally across the opening, where the straightedge rocked back and forth. At first, it seemed very large, out of spec for sure. I borrowed Spouse's feeler gauges, and learned that the rise is tiny, well within spec. The high part was near the front of the opening, and it made the fulcrum for a "large" gap at the back of the saw, where you could easily see it raise and lower as you rock the straightedge back and forth. When I actually measured the gap at the back, I was surprised to learn that it was only ~0.012 inches, meaning that the rise at the front of the opening (the fulcrum) had to be tiny, but was magnified in the back of the table. As soon as I understood that, I moved on.
I loosened the bolts attaching the two wings. I reworked the seams, and they are much better now, close to perfect. Once I had the seams optimized and the bolts fully tightened, I reattached the front and back fence rails to the main table only, and aligned them with a depth gauge.
I used the new 48 inch straightedge to align the top. Thanks to my previous measurements across the blade opening, I knew that the smallest gaps were far smaller than the 10/1000 inch spec for the table and 20/1000 inch spec for the wings. I used the straightedge as a guide. I used 10 inch handscrew clamps and my own muscles (both lifting and pressing) to "tweak" the outside ends of the cast iron wings as I tightened the wings to the rails. When all was done, I put the straightedge across the full width of the table and wings on the front and again on the back to check my work. I saw tiny ripples in the cast iron with tiny light gaps in a few places under the straightedge, but it was as flat as I could get it, which is all I wanted. It took between two and three hours to redo the work to my satisfaction.
The cast iron table and its wings are as seamless and level as I can make them. They are not perfect, but I never expected perfect. I know I did my best, and that's what counts. I spent way more time than @FrankC
would have considered reasonable, but as I said before, it was my time to spend. I do not expect to touch any of those bolts again in my lifetime, except to brush off the sawdust from time to time. :-)
I ran out of time yesterday to complete the saw assembly and alignment, but will finish it soon.
Thank you once more for everyone's advice and suggestions.