instrument cluster
 

the cluster in my blazer has been taken off by previous owner who said id doesnt work i dont wanna put it back in and find out it is truely busted because he has taken all gauges out except spedo i was wondering if there is a way to test w/o putting it back together

With a wiring schematic all you would have to do is test the traces from the connector on the back of the cluster to all of the termination points where the gauges hook in. One thing that all of the K5 trucks have in common is the somewhat crappy contacts used from the flexible circuit board to the gauges. These contact corrode over time and the resistance increases to the point that the gauges are not accurate. The good thing is that everything can be taken apart and cleaned up. You just have to be a bit patient with it. I had to redo a trace on the flexible circuit board in the cluster for my '74. A little solder and a piece of copper and it was done!

so would it be better to fix and put the stock gauges back in or modify it and use aftermarket gauges?

That is entirely up to you. I actually like the layout of the stock cluster.

If you were to go with aftermarket gauges, I would suggest getting a blank gauge panel and doing it all the way.