Pentax C mount is probably an SLR lens.
The optical focal length of the lens is not necessarily related to its physical dimensions. For example, most SLR lenses are retrofocus designs, so that they are longer than their optical focal length in order to provide room for the mirror box.
In fact, my 28mm and 50mm lenses for my OM1 have the same physical focal length (the exit pupil is the same distance from the focal plane).
You can test how far from the lens mount the focal plane is for a given lens just by pointing it toward a scene at infinity and holding a card up to the rear of it until the image projected on the card is sharp. It works well at night, with distant lights.
That lens is only going to throw about a 1" image circle. Is that enough for your tube?
Ah, okay, so the physical focal length might be "folded up" somewhat, depending on what kind of camera it was for. So there's a focal length the light path travels, and a practical one if you stuck it on a ruler. This was for a CCTV or television camera/professional grade camcorder kind of thing.
I think 1" should be really good for the tube. More is better, otherwise I think it would (vignette?) around the edges of the tube's output to the eyepiece screen. Actually, I believe the input fiber optic bundle for the photocathode actually is 25mm so 1" would be about perfect. From all my reading on DIY builds, they stress you can always dial the aperture down on the light, but when you need to be maxed out, like on a moonless overcast night, you can't magically add more.
A question, what do they mean by "format" when they say a lens is 3/4 format? I'm not sure what this lens would be. Does that have anything to do with the image circle at focus?