Thanks, I too think it turned out very well. The camera has a minimum of 5" and a maximum of about 32" of extension. It might have a bit more length, but I ran out of rails. It weighs 13 pounds without a lens and is very, very solid and rigid. I don't know how much front rise I will need either and am thinking of adding 3-4 inches which I think has to be more than plenty. I'm assuming that because of the panoramic aspect that I won't need too much rise, but time will tell.
You mentioned wanting a third arm to use a loop with long lenses. Do you wear glasses? I got a Bausch and Lomb 3.7x magnifier that clips onto my glasses and works very well. It allows me to have both hands free to operate the camera.
Have you thought about a tripod head? I first tried a Gitzo G1570M and it bounced like a diving board. I went with a Ries and it is a rock. It of course also weighs as much as one. Good luck on your Franken ARCA and please post some photos of it when you can, it also sounds like a wonderful way to go for ULF.