My first thought was the converter wasn't in all the way and engaging the pump tangs.
Been a long time, but does the converter 'drop in' twice to get to it's final position? First time it engages the splines, second time it engages the pump tangs.
Or the converter is ballooned.
There are different Nailhead starters, but I've only dealt with the 65-66 ones.
Flex plates are the same, should have two bolt positions for either the large 'big car' converter or the small GS one.
Wheelz, with the converter installed in the trans, can you put a straight edge across the bell and is there space between it and the converter?
I might be able to check one of my trannys to compare yours to.... if I can get to it. The weather here still sucks.... rain today, and chance of frozen stuff tomorrow morning. .... and freaking cold again for Saturday, ugh!