The recoil spring doesn't provide tension, that's right. What it does is sets in the track frame with the rear end against a support that keeps if from moving rearward, and has a hole in the middle that the end of the through bolt goes through to keep it from jumping up and out/away from the support. (((I fabricated and welded up the whole setup on both sides of the one I redid as they were both rusted away to the point of being bent and unsafe))) On the front, the assembly sets on two ears that are captured between the frame and two bolt down plates that allow the spring to compress and move rearward should something get into the track.
That being the case there is a given distance between the rear support and the place the two ears set that has to be right or you can't put the assembly in and still bolt the hold down plates back on.
That being said, what dependzic says is exactly what I remember the manual saying. I wish I had it close but the manual I used on the 977 belonged to the customer. Further, the way I remember it, what you say is true, there is a specified length to the assembly. The way it works though is that the length is given to insure the assembly will fit back into the spot where it sets without being too long or short, nothing more. If it's off either way then the hold down plate will not bolt on and you have a BIG problem. From there there is another spec to back the nut off and then use the center bolt to lock it in lock it in place. Believe me, if one is put toether too short or too long it is BIG and a royal PITA. The two assemblies I had to install were both, thankfully, or unfortunately, given the point of view, way too short, and not too long. If they were too long I don't know what we would have done because there would have been no good way to hold it to tighten the nut. That would have been a BIG problem because to back off the nut it being under so many tons of force, took me and another guy, using a 24 inch adjustable wrench with about a 8 foot pipe handle, nearly an hour per side to back off the nuts and get everything right.