I was testing a Gundlach 36 inch lens I just refurbished, and decided to compare it to my Nikkor T 800mm f12, which is convertible to 1200mm also. I was using my new larger 11x14 camera, which replaced an 8x10. I did not expect the Nikkor to cover, since the published coverage is only 305mm. To my great surprise it covered easily, even wide open.
What a great find this is, because not only is it a great long combination, but the telephoto design means the bellows extension is much less. The camera is much more stable, and the bellows not maxed out, or beyond the limit of extension. The 1200mm is lke having a 48 inch lens, which would have been impossible otherwise! Yet the required extension is only about 31 inches. Any one else tried these lenses on larger formats?