If push comes to shove, use a stock flex plate but prep it by sanding all evidence of sheared metal off of it and sand out nicks, gouges, etc. Then have it cryogenically quenched or shot peened. The trick with shot peening is to peen both sides at once in an automated machine to keep it from warping, OR a little peening at a time to each side until you have 100% impingement. Then do it again for exactly the same amount of time. Cracks always start from holes, nicks and gouges, so smooth blend ratios are really important, so at a minimum you should peen the hole interiors and any repaired defects. I think I would avoid peening the welds excessively though, but I would definitely peen the heat affected zones around the welds. You can easily double the crack and tear resistance strength by peening.
