A top tether helps with minimizing head excursion.
It was my understanding that GM (who makes Pontiac) had a solid policy of retrofitting for free, so your dealer SHOULD retrofit for free. you might have to get uppity with the dealer, because some of them don't necesarily like to do things for free, but they SHOULD do it. You can always contact GM for help if they do not.
The transport has poor crash test ratings. Because of this, I would want my child to have as much protection as possible from the carseat, since the van itself isn't providing much. but then again, I underastand not having any other options, we have a geo metro, with similar poor ratings, that we have to drive for now.
Good luck with the dealership!