How close to actual GNX dash are you looking to get?
If you are just trying to upgrade the pathetic offering that was the buick analog dash with some easier to read functional gauges, then I would go with one of these kits:
They have all of the necessary gauges in a nice matching set minus the boost/vacuum. It looks like you have room for 6 smaller gauges and the 2 larger tach and speedo gauges. I would try to find VDO gauges to match what you have already started there.
I work for a goverment fleet and we have boxes of misc crap left over from surplus vehicles that went to auction. I found them in a box of misc lights and harnesses that was bound for the dumpster. I wish I had more to go around.