The top cover didn’t come off easily on mine. Once I removed the 2 screws I had to slowly lift it up from the front until it popped off. It looks like there might have been a small glue spot at the back center of the plastic edge.

Just did this with my 2007 Mac Pro and the steps are the same.

I used straight SATA cables (since I had two lying around), which get bent a bit by the fan assembly. It should work ok if you're real careful with the assembly steps, but if you have a choice, get a SATA cable with an L-shaped end as shown in the guide.