Generally the crank then flywheel or flexplate-now there can be what seems to be a spacer OUTBOARD of the flexplate, not behind it. It reinforces wheel there at bolt holes to prevent cracks.
What makes you think something else goes there?
The converter when all the way back must just clear any studs if present on that one, you then pull the converter forward to engage studs or bolts in holes. Meaning converter can seem awful far back, like too far. If flywheel, converter, trans case all correct you simply pull converter forward and button it up to plate.
I've never seen any sort of part bolted between crank and flywheel/flexplate on any engine I've ever worked on, engineering frowns upon that, it makes the bolts much weaker and prone to break.