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You can use the stock flexplate with no issues, but I'd check it for any cracks. Fwiw I changed mine out when I put my converter in because my original flexplate had 220k+ miles and I replaced it for piece of mind.
looked at your profile,a 99 Z28. Flexplate problem starting in 99/00?,don't remember the year. Flexplate with 'peanut' shaped holes were prone to cracking,prior to those the one with round holes were OK.