Getting Ready for the Magnum Swap. (help?)

The Magnum head cracking issue is definitely exaggerated. The deck surface is relatively thin and flexes a lot, and cracks often develop between the valves. The cracks almost never cause a functional problem, so if you get a good running Magnum engine you can do bolt-ons like camshaft and intake without removing the heads.
The only real concern with cracks is if you must have ported heads or if your engine has a damaged valve seat you would not want to dump money into those heads. That's when it's time to buy aftermarket heads.
Of course if the budget allows, definitely do aftermarket heads for the performance increase.