1999 Chevrolet Cavalier Recently Put Low Mileage Engine 99 Seem Haveing

i recently put a low mileage engine in my 99 cavalier and i seem to be haveing a circulating problem with my radiator. it boils in reserve and then spits out of overflow. things done new therm stat. and bleeder to eliminate air is broke off wondering if i have a real bad air block or may my radiator be plugged.