I need to know what the distance is supposed to be from the flexplate to the rear of the block. I'll take any of the available measurements: crank flange or ring rear to bellhousing mounting surface or some other point on the back of the block.
My starter gear does not engage the ring gear of the flexplate all the way. There is no adjustment in where the starter mounts and the gear does move out all the way to the brass bushing on the end of the shaft, so the problem is not obvious at the starter end.
The crank is all the way to the rear of the block (I don't know how much movement there is front-to-back) so I know this is going to make the problem seem worse, but I don't think there is supposed to be enough movement for me to question the distances.

Thanks for the help.

Are you sure you have the flexplate installed the correct direction? There's a lip on the center hub of the flexplate, and that should be toward the transmission.