Steve, Yes, the problem is all down to the non standard speedo. The original chronometric is shallower, and has the cable and panel lamp holder offset. GB's photos show what it should look like. Your non standard speedo is deeper and puts the cable slap bang where the back of the original headlamp shell will end up. To do it properly will cost a lot, with a genuine chronometric, or dirt cheap with an Indian repro. Sacrificing your original shell with a bit of backyard fabrication to clear the obstructions is a good short term fix while you decide what to do.
Swarfy.