Don't know if this is of any help, but it will bump your post back to the top.
I went looking for a particular material for my oil pan gasket ('79, 31 a few months ago. Three different parts places said it was only available for 1991's and up. Makes me think that there was some sort of design change that happened in 1991.
It appears that is the one swap that won't work, though I've yet to see exactly why, and I haven't had the opportunity to do a side-by-side on a situation like yours. A 360 LA pan will work on either of the Magnum engines with the appropriate gasket, and the Magnums will swap with each other with proper gasket, but it appears you're outta luck.
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