Magnum head porting, worth the cost and effort without cam swap?

I wouldn't spend the time unless you got a different cam too. Here's an idea though... If your confident of your porting, you could always port the heads, swap them for a factory magnum set and a little cash, and use the extra cash to buy a cam for the new engine. Although I'm not sure you would make enough profit from that, cause something in the back of my head is telling me the newer engines had the roller lifters, which is $$$.