With regard to the Allpar page, take note of what source they give for their 'four reasons' it was built as a slant...
"...according to a 1960 press release." it says.
The reasons given are:
1. Lower centre of gravity. Well, it says 'much lower than in upright engines.'
2. Lower overall height to enable it to fit into the Valiant.
3. 'Engine accessories are more accessible.
4. The water pump can be moved to the side of the engine to reduce the engine's overall length.
As I've already said, the last one is poppycock, the water pump could have been moved to one side no matter what angle the engine had from the vertical, or even if it was vertical. I'll see if I can do something during the coming week about completely disproving the height issue, the centre of gravity is a little harder to prove.
The next line from the Allpar page is, however, very telling:
"While the press release did not mention it, the most important advantage might have been making room for an unusually efficient intake and exhaust manifold."
Gentlemen, having spent years as a motoring journalist, I can tell you that what press releases say are more inclined to relate to what the public would like to hear and what they might swallow rather than actual facts.