Pull the valve covers and inspect for a broken spring or loose rocker. Could be a bent pushrod too. Pull the pushrods and roll them on a mirror to check for flatness.
I'm using the stock oil pressure, voltage, and temp gauges by fitting the Datsun senders on the LS block. It's really easy. Speedo and tach are Pro Comp's.
Sounds like it got over torqued. Bolt stretch is what keeps it tight. Once that is gone there is no force left to hold it and the threads get buggered up. Personally I always use new bolts on engine parts. Some of that stuff is torque to yield and can only be used once. Drill and coil time.
Put some aluminum anti seize on the bolts and torque to the proper number.
Put it up on 2 X 4's like said then put on the emergency brake then jack up the front and put jack stands under it. Then go to the back and jack it up and put jack stands under it. If you dont put the stands on it could roll off the jack and smish you.