8" hold up surprisingly well, and Auburn gear makes a cone clutch posi for it, lots of ratio choices, housing and axels bolt right in, common. Prolly depend how hard you plan to push it HP wise
Yep. The days of finding a $100 9 inch, especially one narrow enough for a '65-'66 are long gone. By the time you buy a housing and axles ($700 and up) and a limited slip 3rd member ($1000 and up) and brakes (used drums if you like) that 8 inch Auburn will seem like a bargain and will be just fine on the street.
I narrow my own 9 inch rear ends ,still cost around $75 for a wrecking yard housing to narrow ,depending on how narrow you go you can reuse your axels from your 8 inch . There were two axles used in the 8 inch though ,same spline but smaller shaft in the middle ,you want the thicker axles. 8 inch and 9 inch share the same axels ,unless it is the big block 31 spline