The various Chinese made 617 backs for 4x5" cameras are extension backs, so they do work, but there are some limits with minimum focal length due to the construction of the camera and how close you can get the lens to the focal plane, and maximum focal length, where you start to run into vignetting. Ian's point about the extra extension from the FP shutter on a speed are valid. I'd start with a 135mm or 150mm lens and experiment from there.