First thing to do is verify TDC using a piston stop. See if the numbers on the dampner are correct.
I also have that cam in my 383. With my modified advance curve, I set mechanical timing at 36*, all in by 2600 rpm because I know I am getting some centrifugal advance at idle with the soft springs. I never set or pay attention to initial timing, only use total mechanical timing.
BTW, I use an adjustable vacuum can that gives me 14* @ 12". This gives me 50* at cruise rpm. I have the can hooked up to the manifold vacuum port, not the ported vacuum.