Keleo makes an exhaust for the Hanger 9 P-40. It's very similar, inverted mount and is made for OS91, Saito, etc. I'd thought about approaching them to see if they'd make one for the P-6. They would just have to add 12 exhausts per side and adjust for the pipe exit location on the cowl.

I guess it doesn't hurt to ask! To not ask is to not receive!

Steve