if I find a posi unit. Can I simply pull out the old one and put the new one in? without messing with anything else?

No, you will have to reset the backlash and bearing preload. This is done with shims.

------------------
'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)