The truck has been held up somewhat while various parts arrive. I've gone to a bitz site for the Land Raider parts that serve as part of the bonnet (there's just a couple, so might as well get it right), but I'm now going to need to have those arrive to get measurements off them.
Still, a bit of progress has been made.
Carefully studying the original pictures, the cut line for the cab is visibly just behind a small square notch in the side (actually for aligning construction, but hey, it's given me a good reference). This also lines up with a panel gap in the roof, so it makes for a logical cut in general:

The cut was a bit of a nuisance, as it can't be correctly supported from the side where you can see where the cut should be, so it done by repeatedly scoring along the line, until such a point as the line became clear on the opposite side, making it easiy to do the last part with it flat on a cutting surface:


Annoyingly, some of the casting marks that would normally be hidden are going to interfere with where I need to install the back of the cab, but I removed them with a sharp chisel and a riffle file:

However, in the meantime, I've managed to find one of the OOP searchlights in my own bitz box and get good measurements off it to make a model for 3D printing.

The plan is to print this in clear resin in order to fit it up with LEDs. (Itself another challenge, as there's not *quite* enough space in the rear chassis to fit in an AA battery box, but an AAA box is just about clear of the right mark):
