126bis Ignition Switch replacement

[Abarth982]

I've got to replace my ignition switch and thought it would be easier drilling out the two security bolts that hold it to the shaft on the bench rather than in situ.  However, it looks like the upper mount for the shaft is bolted to the bulkhead with two (normal) nuts and two security bolts (why?).  So, what's the best way to tackle this, I'm erring on just drilling the security bolts for the ignition switch in situ?  Many thanks.

[Abarth982]

Just in case anyone else needs to do this ...
The security bolts on the mount are pretty flimsy so a soaking in Plusgas or similar will allow you to undo them with decent pair of molegrips. 
Then much easier to replace the Ignition switch on the bench.

[Minijohn]

The shear bolts on the column are for security.

If you remove those bolts, the steering column complete with lock attached can rotate and the car could be moved without releasing the steering lock.

I changed my steering lock recently and found the easiest way was to cut a slot, with an angle grinder, in the head of each screw that holds the lock to the column, then just unscrew them with  a big flat screwdriver