Called them, again.
The bell housing included with that kit, while basically a SBC pattern, is modified for the starter (notch in the picture?).
Speaking of starters, a stock starter will NOT work with their adapter plate.
Plate is 5/8” thick. They emphasized again and again it was designed for their 2 speed bell housing, and that modifications would be necessary for 6L80 and especially getting the flex plate right for 6L80, the spacing was different.
The tone of the conversation was basically that the kit is for their 2 speed race transmission and stop bothering them. They had no interest at all in other applications.