318 Build Up

You can use the Hyd. cam in a roller block and the Magnum heads with a milling. No problem, but you'll need to re-measure pushrod length (Not hard) and get the proper pushrod ends, ball and cup up top.
Also mill the manifold face proper (Specs in the MoPar engines book or a search here) fit. Possible bolt hole enlongation on a heavy mill. Not a problem with a drill and hole size bit or dremel/die grinder.
I'm guessing the 273 is OE for that car? I'd go with a pocket ported 302 headed, roller 318 in a heart beat and lie to everyone. "Yea, I'm tellin ya , it's a 273, it rocks! I can't believe a little MoPar make this kind of power!"