Polygonal Key (Wrench) for GTX Limited tuned pipe

[stevexp2]

I am trying to remove the tuned pipe from a 1998 GTX Limited and the shop manual calls for a special tool - a polygonal key - I think it's actually just an offset wrench and it's to get the one nut that is right underneath the manifold above the carbs.

Does anyone know the metric size for it so I can go and get one from the autoparts store?

Thanks!!