[ray_mcavoy]

Yes, the cluster shown at the top of your photo looks like a 76 or 77. It has a mechanical oil pressure gauge and a voltmeter.

The cluster shown at the bottom of your photo looks like a 73 - 75. It also has a mechanical oil pressure gauge but an ammeter instead of a voltmeter (as can be seen from the back by the difference in the terminal orientation).

A 78+ cluster will look very similar from the back, but will have an electric oil pressure gauge. So it will have electrical connections (that look similar to the back of the temp or fuel gauge) on the back of the oil gauge instead of a tube fitting.

The fact that you found a 76-77 cluster in an 80's truck isn't all that uncommon ... the tach clusters seem to be a fairly common swap.

As for the back of the tach itself, they all look very similar as well. Although the 73-75 ones have a different meter movement that uses 4 round studs w/small nuts on the back vs. 3 studs for the later years.