Sorry to join in late. BSA Bob is right, it is a misalignment problem. I have seen this many times but it does not usually progress as far as this example. Either there was some dirt or burr on the crankshaft hub or flexplate preventing it from mounting properly. Or, more commonly, the transmission was not in alignment with the block. Think of tilting the transmission at an angle to the block and not having a U-joint, Something has to give! There could have been dirt or a burr on the engine/transmission mating surfaces. The locator dowels in the block may have had dirt, rust, or a burr which would keep the transmission from pulling in totally flush. Also, and this is not as uncommon as you would think, the alignment dowels on the back of the block and/or the alignment holes in the transmission mating surface are actually out of spec causing the mis-alignment. Check http://www.wilcap.com/ALP.PDF
to see how to check alignment or google "transmission to engine alignment", "belhousing alignment" etc. I know they show manual transmissions but it gives you an idea of what to check for. Any speed shop can get offset dowel pins to correct mis-alignment and Harbour Freight has a cheap dial indicator and base that can also be used for many other things such as crankshaft end play, setting differential gears etc. Because the cracking, and in your case, the eventual total failure of the flexplate takes quite awhile to happen, the flexplate is after all "flexible", people have been known to just replace the flexplate and solve the problem. If it was just dirt and/or a burr from a previous sloppy installation and this was taken care of during the new installation, then the problem is indeed fixed. However, if there is actual engine/transmission misalignment and this is not corrected when the new flexplate is installed, then the new flexplate will also fail somewhere down the road. As a side note, any actually mis-alignment will also strain and or fail the front transmission bearing. In either case we are talking about a fair amount of time here, it won't happen over a couple of weeks or even months, depending on usage.
I would clean and check all mating surfaces and the alignment dowels and then just to be on the safe side check overall alignment. As far as the engine goes, you have lots of good advice already. Good luck.