Some good data here.
https://paceperformance.com/p-28983-...imensions.html
http://www.novak-adapt.com/knowledge...automatic/6l80
I have a high hump '64' with Early Classic engine and trans mounts and still have 3-4 inches of clearance. The 6L80 looks a bit larger in daimeter than my 4L60E but I can't imagine I would have fitment problems.
Nice having 2 extra gears, but it is not as strong as the 4L60 or 4L80.
They came out with a 6L90 shortly after the 6L80. Maybe this fixed some of the issues they had with the 6L80?
Good luck.