[FWB]

i'm skeptical with 20 thou on the motor its the cam at all......crate motor? what type?
a complete factory drop in for a particular year? a performance engine? does this motor have an EGR plate on the intake? the 70's 460's suffered symptoms like you describe when the EGR went bad.....it acted like a huge vacuum leak, and yes it could knock too.
a little more info on the application of the motor and some build info may get you some more definitive answers

fred