manifold questions

iv got them on a 360..the guy i bot them from said they were from a 340..the book says 71 duster or dart 340...im trying to get some info on these from the exhaust gurus..how do they work out on your 318...the actual 318 manifolds that came on these cars are somewhat smaller in port size..but look the same.im trying to find out what the mating number manifolds are .that 318 should breathe better with these on it,..jim