Don,
A good friend of mine had problems with one of those little starters on the Model-A he is building. In his case, the starter fit too close to the block and wouldn't engage. At first he thought he had the wrong flex plate, but a count of the teeth proved it was, indeed, correct. He tried shimming the starter farther & farther from the block, but ended up with an ugly stack of spacers and it still made nasty noises whenever the starter engaged. He finally determined that the darned thing was just machined wrong and took it off. He put an original style starter on there and it works perfectly with no shims or other modifications.

My point is, the problem might be with the starter and not the flex plate. While you are measuring things, measure the snout length and mounting holes on the starter. It would be helpful if you have a stock starter to compare dimensions with, or, better yet, bolt it in place and see if the teeth disengage properly...