Magnum heads on a 273?

They will fit. However you would want to put some reliefs in the top of the cylinders in order to unshroud the valves. Easiest way is to mount the heads on the bare block and mark the areas to be relieved from underneath - ie from the crankcase side of the cylinder bores, most likely the intakes would need the reliefs more than the exh valves. What you would be making are some crescent shaped ground out areas at the top of the cylinders. I have ancient mopar speed books that talk about doing this with similar valve sizes.