That's an interesting article. It lends weight to the whole idea that a significant part of the slant's slant was to give room for an efficient manifold. This is mentioned, as I posted previously, on the Allpar page referenced by 64ragtop.
I get the impression that many are not so impressed by my reasonings and suggestions, so I've used the sectional drawing of the slant from the Allpar page to make a composite picture that shows the slant slanted at 30° alongside the engine brought back up to the vertical.
You can clearly see that the height of the engine effectively doesn't change. Critically, the ports are at the same height and so the carburettor, the highest point of the engine, is committed to the same altitude as it would have been with the engine vertical.
The importance of raw engine height is further degraded by the fact that the manifold runs uphill from the ports. And my second pic, also from the Allpar page, shows clearly how the air cleaner is several inches above the height of the valve cover and would have remained so if the engine had been upright.
So we have a PRESS RELEASE that says that engine height was a reason for the design. This myth, in my view, is busted.
As far as the slant being to allow the water pump to be beside the engine is concerned, that's another product of the fertile minds of the promotional people. It's not hard to find vertical engines with the water pump beside the block.
Lower centre of gravity - that is certainly the case. I would think the centre of gravity of the engine would have been lowered by anything up to an inch.
The fourth item on the press release related to accessibility of the accessories. As has been pointed out, distributor access isn't good, but pretty much everything else benefitted from the slant.
Another point in favour of the manifold efficiency being a major factor is the need for the manifold to avoid clashing with power boosters and brake master cylinders.
To further emphasise this, I will point to the upright design of the Hemi 6 engine with its similar manifolding. In Australia the power boosters and master cylinders are on the opposite side of the car so there was no need to cant the engine. That engine is larger than the slant, it sits under exactly the same bonnet (hood) as the slant did in the same car but it is longer because of its bigger bore. Even so the water pump is not offset, but the engine bay was lengthened to cope.