Yeah, the chain has a tensioner that runs off oil pressure. You check the chain itself by measuring the distance between links, you'll want a micrometer. Harbor Freight has a cheap one that will suffice.
The chain should never be loose on the drive side (right side, as you face the engine bay) unless you've rotated the crank shaft backwards. There will be slack on the opposite side, and that's what the tensioner is there for, to take up that slack. If you don't have steel guides and don't know when the timing set was replaced last, I'd make a point of doing it next time you've got the cover off.
Crankshaft bearings I'd call a solid 5 out of 5, just because of the consequences if you get it wrong. It is possible to drop the pan, unbolt the caps, slide the bearings out, and slide new ones in. I don't recommend it, because the precision measurements needed to get the right tolerances are hard to achieve flat on your back under the truck with oil dripping in your face. The right way is to pull the motor and put it on a stand, and if you're going to do that you might as well rebuild the entire thing. Which isn't a bad idea, especially if you can scrounge a spare block and work on it at your leisure.
But I agree with Hyde, I think you're borrowing trouble at this point. See if you can borrow an oil pressure tester, just to verify that you do have pressure. Once you've done that, test your sender and if it's good, check for the same signal at the wiring on the back of the cluster. If that's good, replace the gauge in your cluster (never done that, might be easier to replace the entire cluster?).
