The "sub board" as you call it is the voltage stabilizer, which provides power to the fuel and temperature gauges if it is like the earlier ones that I am familiar with. There are two terminals on it, labeled B and I, and each terminal has two spades. There is one connector already on one of the "B" spades, which has two wires coming out of the connector. Those two wires are already connected to two other terminals at their other ends. I would assume that those are for the fuel and temperature gauges and are already connected in the proper location, and the other terminals next to those two (N1 & N4) are for the wires that go to the "senders" for those gauges. The "I" terminal should be connected to the wire coming from the key switch that is hot whenever the key in in the "run" position, and that wire should have an in-line fuse holder with a 5 amp fuse.
Keep in mind that if the new instrument cluster is different from your old one, that the resistance ranges for the senders for the fuel and temperature gauges might not be correct for the new gauges, so they may not be accurate unless you get senders to match the new cluster. I know that applies to the earlier ones when you buy replacement instrument clusters these days because they changed the senders over the years, but yours is a later model and might have had the later resistance value senders from the factory.