Sounds like the converter has the LS bolt pattern of 11.066". The OE 4L80 flexplate is 11.5", thus your ~1/4" difference. Like mentioned above you can drill the flexplate, as Hutch mentioned they need to be accurate - preferable on a mill or with a centering jig. You could also get a used flexplate from a 4L60 or 6L80 vehicle and it will fix your problem, you just need to ditch the spacer between the crank and flexplate. Since the converter has the LS bolt pattern, I am assuming it has the longer pilot for the short LS crank.