That's a good question, I'm pretty certain I've seen some serpentine belt set-ups in Panteras. I'd be interested too in knowing the details. I haven't done this myself, but I can at least mention the one "snag" in doing this with a Pantera is that a 351C mounted in a Pantera chassis requires a smaller diameter crank pulley to clear the frame. For instance the set-up in the picture below wouldn't work in a Pantera.
There are lots of threads regarding this conversion over at the Clevelands Forever Forum, to find them just type "serpentine belt" into their search engine.
-G
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