[hatzie]

One thing I failed to mention is the fact that the later clusters have electric oil pressure senders while your 74 has a Bourdon Tube oil pressure gauge with a copper line running through the firewall to the engine.
If you were moving from warning lamps to gauges you would already have an oil pressure sender wire. You have gauges so you'll have to route an oil pressure sender wire. You can likely utilize the open oil warning switch/lamp Circuit #31 spot in the bulkhead plug with Delphi Packard 56 male and female terminals on SXL wire. That spot should be unused on your gauge cluster truck. If you want to be "correct" about it use Dark Blue wire.
The 8900371 instrument cluster plug is obsolete and likely the terminals are as well. A terminal and wire pigtail will need to be salvaged from a 73-87 GM truck.