Yes you need the extension. there like 3$ from autozone.
And unless your running mufflers your universal x wont come close enough to the rear seat belt holes. My shop hung mine by the old hole used by the stock exhaust hanger under the tranny.

 

So why do you need the extensions with pacesetter headers?

 

i didnt use any extensions on mine when I did them. I used flowmaster 3" to 2.5" merge that would slip over the end of the pacesetters. you have to trim like an inch off the one side to get the X to meet up right.