5.2 magnum swap--which gasket do I need?

You are correct, pans are the same on magnums, gaskets are not. The 5.2 magnum has the bigger rear main just like the 5.9 magnum, for what ever the reason the gaskets are not the same, still not sure the difference but I know its not because it has the LA 5.2 style rear main. So a 273, 318, 340 pan will not fit either magnum.