Old GT40 heads maybe? The C6FE threw me, I never come across any becasue they're aluminum, meaning not a regular production head for an FE engine. So the GT-40 performance platform makes sense.
Over on Club Cobra Forum they were mentioned about 6 years ago, someone said they're not all that and everything aftermarket now day's far our performs them...
According to Tom kirkham "Aluminum MR heads were used on the GT40 MKIIa and other lightweight engines like the 67 GT500 Supersnake."
That's all I can offer. I'll give ya $25 bucks each for them. We can go halves on shipping.