Factory 318 is same port size as a 273. 340/360 share the larger port size.
So depending on which heads you actually have, any 273/318 or 340/360 header for your year Dart should work (provided they will accommodate power steering).
I don't quite follow what you have for a transmission (cable shifted 727 for smallblock I think?), but be aware of convertor register differences on pre and post 1968 crank flanges and convertor snouts... I'm not aware of any aftermarket shifters anymore that accommodate the early cable-shift transmissions.