340 Exhaust

OK I need some expert help here.I am going to be installing a new 416 engine in my 69 Dart Swinger 340 soon.I am not fond of headers and I prefer stock type cast iron exhaust manifolds.I have been told by a couple of guys that the magnum exhaust manifolds will fit a 340.I am going to be running a set of original built up X heads on the 416 and I found some magnum exhaust manifolds and I like the looks of them but I am not sure of the fit or performance.So I guess my questions are!! Will they fit X heads and if so will they kill the performance!

Thanks

Bobby