I think Stan used the VW engine and the dune buggy because it allowed a simple and representative application of the technology to an automobile, while leaving everything exposed and easily accessible, nothing "under the hood", since the dune buggy has an exposed engine.

He shows his systems being installed on several Aircraft and high performance race cars, so the specific engine type is not critical at all.

Here's the general math for how much HHO you are going to need.

Water is 2.5x as powerful as Gasoline.

If your engine gets 30 mpg at 60 mph, then you are using 2 gallons per hour of gasoline, so you need 2.5x less water which would be 0.8 gallons.

When you split the water, it expands roughly 1800 times, so when you split this 0.8 gallons of water, you'll have 1440 gallons of HHO per hour, or 24 gallons per minute. (91 liters per minute) ... based on this example from my initial assumption of 30 mpg at 60 mph.

Now you can do the math to find out how much volume of air fuel mixture your engine needs, considering the HHO requires no air for combustion, the rest of the volume is extra ambient air and recycled exhaust gasses.