I did one and used a small rubber hammer. It gets real hard to get it in the back corners. The corners are very tight. We really didn't have much trouble except those two spots.
I run a Hydromotive on my 03 and couldn't be happier! I know several that run them on their 21's also with great success. I would be calling Todd at Hydromotive! You will not be dissappointed!
That piece in the pic above is upside down? It has two things sticking up off it as it is in the pic. If you turn it over, the pin goes in the hole in your deck and where does the other piece go? Look at it closely, see what I'm saying?