It's way more fun to do this yourself in your garage, but one thing you need to be aware of is thaat you re-align the axle-ends to the carrier, and not the cut axle-tubes.
I've narrowed a C-rearend awhile back. Pics are on my website...
http://www.bigblockmopar.com
You need to have/make 4 properly sized spindles and a long strong bar to run through the carrier and entire axle-housing, where you can align the axle-ends on.