Sam, while it has been a while since I have worked on any NP trannies, the fact that the transmission top cover has 4 shift rails would indicate that it is likely a 5 speed.... the input would, in all likelihood, be a 1 1/2" ten spline .... 12" discs are readily available.... incidentally, some of these transmissions were also made with an overdrive top gear, for which, if my memory serves me correctly, they had a weird shift pattern.... as in 3rd gear was the "second" gate pulled towards you, then you went into 4th by going into neutral, pushing over to the "third" gate and pulling it towards you again, the for the overdrive 5th, you pushed it towards the dash.... the reason being that the overdrive gearset was in what, in a top gear direct trannie, was the 4th gear position....