Evaporust contains acid. Acid will eat rust first, but will also start to eat metals after that.

For me personally, I would have no problem at all using it on the outside of a rear end "pumpkin". Since the cast iron is so thick, it wouldn't hurt a thing. I would also soak the gears inside the pumpkin, but not leave it in the Evaporust too long (maybe an hour or two) so that I didn't eat away very much of the important metal inside the pumpkin. My 2-cents.