The top sprocket won't go all the way on. That's normal. I always put silicone on the splines so the oil doesn't seep out the shaft and get on everything. Top and bottom for that matter. When tightening the triangular plate always use a 1/4" drive ratchet if possible or if you use and end wrench don't tighten it to the point of bending the plate or it will leak oil. Don't over tighten. I also use a little silicone on the o-rings/bearings too. Use a straight edge to make sure the two sprockets are aligned with each other when you put in the drive shaft. If not, you will need to move the shaft over to achieve this by unlocking the set screw and lock collar.
The speedo may be rusted as you said. I have replaced one once for a friend that was rusted inside the speedo itself. Had to buy a used one to install. You might get lucky and spray it with PB Blaster penetrating oil. Its worth a shot. Let it sit a day and spray it again.