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Look at the flexplate carefully for cracks around the bolt holes, I'd also take a stone and make sure there are no burrs on the flexplate or the crank..
Whose rotating kit are you using? Eagle specs a neutral balance flexplate. Make sure your flexplate does not have any weights on it. I don't know about the 2-piece seal blocks, but the 1-piece seal blocks flexplate come with a weight attached, even though the engine is internally balanced. It's not easy to find a neutral balance flexplate, Summit had one brand period.
It's no kit.
I have picked the parts, just to know that the weight on the whole rotating asm will be minimized and weight matched.
It's a Eagle ESP crank, (Manley rod's and Probe SRS pistons)
The old flexplate from my 327 looks fine and doesn't have any extra weight.
I will take the flexplate to the guy that will balance the crank tomorrow morning. He said he would do the job early this coming week.
Must be better to have it on the crank than not, even if it's neutral.
One "issue" when balancing the crank may be that the parts does have much lower weight than the parts normally that will go out with this crank.
So the crank will likely look lika a Swiss cheese.