I'm not familiar enough with the GT40, but is there way to fab a simple exhaust, or maybe even find a used exhaust from someone like Fran Hall at RCR or Dennis Olthoff?
Locally, there's a Shelby club member by the name of Tony Navarra of T&D HiPo Motors (lives in Gilroy) and he could source everything. Also, there's Sacramento Mustang and Mostly Mustangs in Oakland owned by Jack Schroll. I'm sure there are a few more locally that advertise in the Mustang Monthly or one of the other Ford magazines.
Mostly Mustangs | Ford Mustang Service, Performance and Customization | Oakland, CA 94612
Or buy the $2K exhaust and then take the hit on selling it, you know "used only once."