This sounds very similar to what I did to a local AULRO member's 110 County, except I just bolted the injection to his existing 3.5 engine.
To sum the job up:
1. I used a wiring harness, manifold, ECU etc from a 1993 RRC as this has a totally separate EFI harness from the body wiring. The ECU wiring is long enough to reach the battery box where I put the ECU and the EFI relays.
2. I used a 1989 RRC EFI fuel tank and fuel pipes as the tank is a simple bolt in fit to the 110 chassis without any mods whatever. Your existing fuel level gauge is wired to the new tanks sender in the original location. The pipes needed a 10" extension near the fuel filter, no big deal. I wired the EFI fuel pump directly to the fuel pump relay placed in the battery box using the pump wiring salvaged from the RRC that donated the tank.
3. To make the EFI run, all I needed was to fit all the parts, hook up one wire to the ignition coil negative, connect the earthing point to the engine block, then there are only two other wires from the ECU harness that enable the engine run. They end at a plug normally connected to the body harness just inside the firewall of the RRC. Cutting the harness plug off the donor vehicle makes it a plug in connection when you solder this plug to the two wires on the 110. You need only tap into the battery positive (fat plain brown wire) wherever it's convenient (somewhere near the firewall) and then ignition positive (plain white wire) ditto. All other inputs are not needed to run this ECU. The speedo input can be ignored, it only complicates matters.
I'll see if I can dig up any pictures and add them to this post later.
Edit: they're all here:
Disco tank into County
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