the 5.9 Mag will be a big improvement over the 273. Friend of mine is running a 5.2 Mag in a 72 Valiant 4dr....stock engine except 4bbl. intake and street demon carb, cheap summit headers - it ran a 13.6 last summer, that´s great. Yours should run at least the same with sticky tires.
Take care of the external balance of the Magnum (proper flexplate), i think you can keep the exhaust manifolds for now. Keep the new cam on the small side, afaik there´s not much Piston to Valve clearance with stock Pistons.
Michael