Per USAF data sheets, the 12"/6.3 Metrogon covers 80 degrees. That's ~ 500 mm. Here's the enthusiast site: http://www.metrogon.20megsfree.com/ If 90 degrees, as people claim on the Internet, 600 mm. The USAF data sheet also says that its AWAR (Super X film, high contrast target, Wratten 12 filter) wide open is 17 lp/mm. A G-Claron isn't it, though, they cover less than 80 degrees. 72 if you're picky.
If you need an 80 degree 300 mm lens, I think there are better choices, especially considering the cost of putting the Metrogon in a shutter.