If you've been using a Gitzo head, is it safe for us to assume, then, that you are using Gitzo legs? If so, then a Ries A-250 head by itself is not going to help you much because there is no way to mount it to the Gitzo legs without losing your ability to pan. You'd be better off to go ahead and get the Ries A100 tripod to support it.
I had a Wisner 11x14 camera myself at one time and used it atop an A100/A250 combination. The large platform of the A250 is good for supporting the large base of the camera. Further, the large crown of the tripod forms a very rigid support for the weight of the head and camera, not to mention torsional forces from wind acting on the bellows. I now have a Wisner 14x17 and am using an early version of the A100 with the early Photoplane head and that works superbly (as long as I don't have to carry it very far.)
This shows the early Ries supporting my 12x20 (a similar load to the 14x17). The extra piece between the camera bed and the tripod head is a "tray" I made specifically for this combination which helps to distribute the load and provides alignment guidance for mounting the camera. Load distribution is very important with a large camera for obvious reasons. Generous supporting surfaces help support not only weight, but also prevent unwanted motion (such as the camera's tendency to rotate on the tripod head). I think the Ries tripods are the way to go for your application. But I'm a little biased, perhaps. There are some other solutions out there. YMMV, as usual.
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