I was faced with the same puzzle today. One side would go on okay, the other was impossible. Then I found that I could rotate the crankshaft (and hence the camshaft) just so to get both sides to fit. Unfortunately, when I finally got everything together, I discovered the exhaust valves hit the pistons so I got to take a break to clear my head so I can figure things out. It could be the cam gear is off one tooth. Or perhaps it's 180° off. I dunno. Someone hand me a beer, I'm done for the night.