Welcome to the board
You have come to the right place for all the answers to your Chevy Truck questions.
First let me give you a link to GMC Pauls great web-site.
The link will take you to the tech tips section where there is a diagram of the changes to the wiring that need to be made if you are switching from idiot light dash to the more common guage dash.
GMC Pauls Tips
I was not sure if your original dash was the idiot light style but thought that you may want to check the wiring to see if the previous owner did the job right.
Like the others said check your grounds.
My 71 has symptons just exactly as you described yours but mine is the idiot light version, I have had mine out and I can tell you that the printed circuit is a real mess.
Have Fun