replaceing differential

[freddavis]

i have 66 chevy nova 2 SS,small block,4 speed,12 bolt posi,410 ratio,with multi leaf springs.
i think the posi is not original,as there is no room for wide wheel/tires.
i would like to put narrower posi third member,with disk brakes and about 273 gear ratio.
there should be a drop in fit from some vehicle.
any suggestions?
thanks
fred