Thanks for your time and your detailed reply. It was exactly the sort of thing I wanted to know and tells me that this would be WAY more of a project than I need right now.

The truck will probably get a horse box back on it and will operate in the 8-12 ton range. The 466 and five speed trucks along at 80-90 km/h very nicely on the flat but hills are VERY slow so some extra power would be nice. Another option is a 500 turbo which I believe would pretty much bolt in. These are available ex military (at a price) but I guess the same axle load issue arises. I assume that going from 466 to 500 (non turbo) is hardly worth the bother?

Operating up to 12 tonnes shouldn't be a problem for the 500 turbo.

A two-speed axle would allow you to split the ratios and keep it on the power-band on hills.

Does that axle give a higher split or a lower split in each gear?

Axles (even single axles) have alternative ratios, and, together with a gearbox that has direct top or overdrive will give you a final overall ratio (and top speed).
A two speed axle offers two ratios, but these can be higher or lower than a single speed according to 'choice'.

For a vehicle such as an artic it would be usual to choose ratios to assist hill-climbing, whereas for a rigid (or a coach) the overall ratios would be chosen to provide an overdrive ratio for higher cruising speed.

So the answer is it all depends - but two-speed axles work by having a single reduction (high ratio) and a double reduction (low ratio).

Again, thanks for the answer. I realised afterwards that the splitter question was silly one because, as you said, it all depends on the ratios in the two speed axle compared with the single speed version.

There were many ratio options available that affected overall speed.
Size of wheel-rims, size of tyres, gearbox 'top' ratio and axle ratios.

A particular model could be specced for a restricted top speed (and better gradeability) or a higher cruising speed at the expense of ultimate gradeability with the number of gearbox ratios and, perhaps, a two-speed axle determining the spread of ratios.

There were so many options that Bedford could produce vehicles for a whole year without ever producing two identical vehicles - though in fact many identical vehicles were produced when customers ordered a batch of identical specification models.

Another (cheaper) option would be the Perkins 540 but I wonder whether it would be too wide. Your comments (again) would be appreciated.

I have no knowledge about fitting the Perkins 540 to a Bedford KG.

You're on your own there I'm afraid unless you can find someone who has done it.

Just remember that fitting the Detroit 6V stuck out the back of the TK cab.

I don't know the dimensions of the Perkins 540.

I'm doing some research but I suspect it won't fit between the chassis rails.

I had a good look at a KM tractor unit with a 6V71 over the weekend. It is a very different beast from a TK. I didn't have a tape handy but I'm pretty certain it is a wider chassis and the V6 is still a tight fit with the exhaust manifolds hanging over the chassis rails. The engine is mounted further back because its width means that it won't fit into the front part of the bay which narrows in the section next to the driver.

Looks like a 500 turbo or maybe a modern turbocharged six such as an Isuzu 6BD1T might be the answer.