Shake rattle and roll

1968 barracuda 360, 4speed. Starts to vibrate at 1800 rpm and gets worse as rpms increase. New clutch, Machined fly wheel, rebuilt trans and rear end. Could this be because fly wheel is not balanced? Also, would a fly wheel have to be balanced due to cast crank? therefore not needed for a steel crank 340? I have heard all kinds of stories. thanx. Paul:violent1: