Problem installing new idler pulley

I bought a replacement idler pulley from napa but had a problem installing it using the existing front and back plates ... the dome on the front of the pulley was different, and front plate didn't fit correctly. I managed to screw up the back plate putting the original back one back on.

http://www.napaonline.com/MasterPag...artNum=12564401

I was using the tutorial at install university, the GM part number crossed referenced to the part I got so I believe it's the correct one.

I wonder now if the front and back plates are needed? I notice on the box for the new pulley it says:

"The bearing in this product is greased and sealed and therefore does not require the dust cover that m ay be used on the original pulley. Check for interference before reusing the original dust cover".

What do you think?

Just an update ... the guy at the parts store said it doesn't use the dust plate, but when I tried it without the rear plate it doesn't line up right ... and of course my rear plate is bad so I had to order a new pulley from GM to get the plates, they don't sell them seperately.