Thats awefully high rpms for a BB. Why so high? As far as diesel turbos go, I believe you would have to go twins for your 390 if your pulling something out of the junkyard. From my reading most guys will run one diesel turbo on a 302 sized motor and even some of those can be small for those motors. I've never delt with turbos, just done alot of reading for some of my own future plans. Heres a link to a thread that might help you out a little more:
Junkyard Single/ Twin Turbo FAQ List