Ran into this last year after changing convertor in a customers car. The convertor fit fine on a 6 bolt crank but on an 8 bolt, the flexplate to convertor bolts hit the hub on the convertor before the bolts into the convertor were tight.
Look to marks in that area of the convertor from the bolt heads. Thin spacers between the two fixed the issue but we used longer bolts to ensure enough thread engagement.
Keith