Just a small update, as the wiring is now through to the rear. Purchased 9 feet of 5/8" (inner diameter) flexible conduit, threaded the loom into it leaving a space at the end, drilled a hole through which a wire was tied and used to pull the conduit/wiring through from the front to the rear. After that, the last few inches of the conduit was cut off and the wiring itself was pulled the rest of the way out.
This should prevent chaffing of the wire loom on any sharp galvanizing edges inside the chassis, if any. I also drilled holes through the conduit every foot or so, before introducing the wiring, so that moisture in the tubing can drain or dry out. I've left the rear end of the conduit protruding from the chassis hole, so that it can be pulled out again if necessary. The front end is lying flat inside the chassis, just under the loom entry hole there, so that moisture, if there's enough, can run out, seeing as that end is 'downhill'.
My only worry was that the tape measure, being flat, could have passed under the bolts that hold the shaft for the hand brake relay lever to the chassis, preventing the conduit from moving through the chassis, but this wasn't the case.
In this view, the conduit containing the loom has been pulled through.
The end result. I've since drilled a small hole through the conduit and used a tie-wrap to fasten the end of the conduit to the end of the grey panel brace, to make sure it doesn't slide back in.