What all do I need

Ok so I found a place to narrow my 8 3/4 rear for 200 dollars.
I need to know all the measurements to tell them what to shorten it.
The said it would be around the same price for new axles then to respline the old ones.
I found these axles are they any good
http://www.summitracing.com/parts/SAG-PA5298/