When the factory installed the SIII ignition switch assemblies on the steering column, they used special bolts whose heads would snap off once the bolts were fully tightened. They did this, because anyone with a Fisher Price took kit could remove the plastic steering column fascia and unbolt and remove the ignition assembly, defeating the steering lock mechanism if normal bolts were used.
The good news is that the special bolts, with the broken-off heads are made of soft metal, so you just need to drill the wide parts off, then separate the ignition housing and turn the remaining threaded part of the special fastener out by hand.
I believe the petrol ignition switch itself can be swapped out with the diesel switch. I don't think there's a need to swap out the whole ignition assembly/
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).