I have also been working on the rear 670 parts. I probably have way too much time into them, but my time is free so....
I purchased the 670 exhaust cover from Marek along with the upper bumper side pieces. This gave me a good starting point with OEM
Dimensions. From there I have been cutting, glassing and adjusting the 670 rear bumper and wing that I got from Inpsire Composites years ago.
I am almost done with the parts now, and excited to body work them. Some of the parts were so heavily modified to fit with the OEM body that they will be used as bucks to pull molds, then parts created from them.
The 670 exhaust cover from Marek is a copy of an FGP cover. It does not come with the mounting points for the 670 wings struts so I had to make small sheet metal molds and glass in the OEM style mounting points. Then I molded the wing struts to match the new flange.