8 1/4 rearend

On my demon there is an 8 1/4 rear from 69 b-body which I got from the part #. It fits in nice I looked to see if it was narrowed down but could't see any evidence. My ? is how strong is this rear and is it hard to get parts for? I wan't 3.55 or 3.91 for gearing and have 340 that I will be rebuilding with projected hp of around 350 t0 450. Any and all advice will surelly be appreciated.
Thanks Matt