There are several ways to do this, but all I will describe is the easiest and simplest.
You will need the 63L tank, the transfer pump & level sensor from a 55L system, late production high pressure pump from a 318is (it has the fuel return fitting), some hoses, connectors for the low pressure pump and level sensor, the fuel filter bracket for the 63L system (optional), and the fuel gauge from a 9/87 or later 325.
There are three minor difficulties, but all are easily dealt with. The first is that a right side level sensor is grounded, but we need to be able to wire the sensors in series. Fix this by enlarging the hole in the pump frame and the mounting holes in the sensor. Use a thicker than normal gasket (or doubled/tripled gaskets) between the sensor and pump frame, plastic sleeving over the studs, and a plastic washer under the nuts to isolate the level sensor from the tank. The second is that the mounting tabs on the left side of the tank are such that the pump frame won’t go in the way we need it to with the fittings pointed to the right. Easily fixed by taking a little material off the sides of one or two tabs (at about 5 o’clock) with a file. The last is that the early gauge expects the resistance of a single sensor. But the 63L tank needs two sensors wired in series and thus a gauge that expects the doubled resistance. Fix this by swapping the gauge in the cluster for a late 325 gauge.
You will plug the return line fitting on the transfer pump and route the output fitting to the 318’s return fitting. Returning fuel from engine bay will go to the fitting on the left side of the 63L tank. The output fitting of transfer pump takes a large hose, but the return fitting on the 318 pump takes 8mm hose. There is very little room above the tank, so use a reducer at the low pressure pump.
The rest is pretty much just splicing in the level sensor and pump wiring, putting the tank in, connecting everything up, and swapping the gauge. If you don’t use the filter bracket for the 63L system you will have to come up with a mount.
And there you go. This system will allow you to run the tank down to a just a few gallons before starvation occurs. At Roebing I ran the tank down to 2-3gal and just got a hint of starvation in turn 9 on the last lap.