Will a 66 b-body 8 3/4 fit without cutting?

Depends on what year the b-body axel is.
Here are the specs.
62-70 8 3/4 b-body perch to perch 44.00"
flange to flange 55.000"

71-74 8 3/4 b-body perch to perch 47.30"
flange to flange 57.800"

63-72 a-body perch to perch 43.02"
flange to flange 53.200"


Perches would have to be moved.
Special offset rims would be needed

I would look for a A-body rear.