Have you registered your Mitchell on our site? It is easy to do and will provide you with more information about your camera. The registration page is the 3rd selection on the top right tabs on the home page. Or- http://www.mitchellcamera.com/registration.html
will get you there. You can find the year for your camera's serial number from the first tab.
Common lens assortment for a GC would be Baltars:25, 35(40), 50 and 100mm. They can be found on E-bay but are pricey. Your biggest problem will be the 4 screw mount. Beware the people selling 3screw mount lenses as 35mm lenses-they are for 16mm.
You can see a 360° stop motion animation of the classic Standard/GC in my videos on Blimped cameras under the "member's articles" tab.
Ken Stone or Richard Bennett or a few other shops like Visual Products could convert your turret to Nikon mounts if they have the time.
For your 2709, that is too classic to not spend the time and money to do it right. ED.