shaking over 55 mph

just recently put new cragars on p215 70 14 tires on front and p245 60 15s on the back on my 73 duster, while on the expressway, not often by the way, i get some shake at 50 60 mph. and steering feels like its floating around some, i know these a bodys do not handle like a new vette. any how, you guys think its alingment-- tires on front not balanced correctly-- bad bushings??? bushings are rubber wth afewsmall cracks in the rubber.