Speedo should be mechaincal. So it's a seperate problem. There should be a fuse for the gages. I think it listed "dash", gage or "inst". Check all the fuses. Use a test light and you can test for power on both sides of the fuse without having to pull each fuse.
I think your going to find problems under the hood and not in the truck. It's either that grond strap or it's lost a Fusable link down at the starter. I'm betting the ground strap.
The bolt in to the head is a 3/8's by 1/2 long.
