Starter/flexplate alignment
Is the battery fully charged? A low battery may not allow the starter to fully engage.
Are you sure the starter is good?
I'm assuming the donor vehicle was wrecked with these:
Is the starter seated properly against the block (not bound against something) or have a cracked housing?
Are the bolt holes / threads damaged on the block?
Are the bolts straight?
I always use GM flexplates that come with the engines or get them from the salvage yard. I've only used one aftermarket flexplate and it was a TCI SFI approved ($$$) which went on a car I was building for someone else because that is what he wanted.
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I too am (recently) having a potential out of round/too large flex plate issue and wanted to see if anyone else has as well?
The starter is hitting the flex plate due to it's circumference (not depth). When I shim the starter down the starter gears engage.
Flex plate is a SFI plate. I wont mention the name until I confirm for sure that is the issue.

