[ElGracho]

There will be some significant wiring changes that will be required to use that cluster in your truck.

Let's look at each gage:
The voltmeter connects to the output of the alternator and a ground in the cluster, not a big deal. There is no "sending unit" but you won't want to use the existing wire that has a switch that closes on low voltage.

Oil pressure: the 75 cluster with a gage would have been mechanical which is why they only listed one "sending unit" for your 75, which is really just a low pressure switch for the idiot light. For the 78 cluster, you need a sender that matches the gage so go to the parts store and get one for a 78. You can probably use the existing wire between the sender and the gage (meter really) if you trace it.

Water temp: again, the existing was a switch and you need a variable resistance sender, just like oil pressure, go and get one that matches the 78 at the parts store. Also, you can probably reuse that wire.

Get schematics for the 75 cluster and the 78 cluster, you may have to rearrange or add wires to the connector on the cluster.