Please school me on exhaust manifolds

I swapped a 360 from a 75 cordoba into my slant 6 75 dart sport using the cordoba manifolds. I took it to a local shop for the exhaust system (friend of a friend) and the mechanic said neither side would work. I will add a holley 600 and maybe a slightly larger cam in the future. What manifolds will work? 273, 318, or ? Thanks in advance...Dean