Is the added safety of your children worth the cost and effort? As far as I'm concerned, you're damned right it is!
I used rear belts out of an 02 Trans Am. You have to fab some mounting pads to bolt the retractors down in the trunk, alter the rear speaker deck or firewall but it's pretty easy. I also changed to the fold down rear seat out of a 4th gen so I ran the belt through the firewall.