Tech's right...

All you are doing when you advance or retard the cam in theory is move the curve up or down 500rpm....

In a heavy car I would advance it....more so than anyone else would but....you have to be careful.... the more you advance the more you invite the death rattle ( detonation) and the 305 is already prone to rattle....

I would strongly suggest degree'n in the cam and run it straight up for starters....