Yeah, the 83 and later have a larger plug than the 76 but still will fit in the same hole. Just more circuits but the 80 and later will not fit the 79 and earlier firewall plug. You will have to change the complete wiring in your jeep.
On the other hand, if the wires on the 83 harness are in better shape, then you could remove the wires from the 76 Plug and put in the 83 wires. The terminals are the same and easy to remove.
And an 80 and later Connection:
Those numbers in the first on correspond to this chart.