You don't need to completely remove the entire cooling assembly.  The block hoses should have just enough play in them so they can be set out of the way.  It may depend on how your power cables are set up.  On mine, they weren't an issue.

Perhaps most important, use some guides to keep the block in place when screwing it back in.  I used 3" wood dowels (chopsticks) sharpened on both ends.  You don't want the block moving around and smearing your paste all over as you screw it back in place.