i traded for a 74 Duster and someone had put a 360 magnum engine in it. it has a vibration. I know nothing about Magnum engines put I do know the flywheel / flex plates are different from LA engines this car is an automatic, before I do anything I was wondering which should it have? the magnum style flywheel or the 727 style flex plate if its wrong could that be the problem?

360 mag and LA 360s are not balanced the same. You need the proper weighted flex plate for a 5.9 magnum to correct the condition. A cheap solution is to use use a weighted flex plate off of a 96 and up 5.9 engine and a neutral balance converter. Although you will have to modify 1 of the torque converter bolt holes slightly to get it to work correctly with your early offset bolt hole torque converter. 96 and up converters have a 90 degree bolt centers.