RE: vvrc 40cc twin
My observation of this engine and the optional availability of the angled 1/4-32 spark plugs on the bottom corner of the cylinders . . . .
Unless it is mandatory for you to have the 7" width, from personal experience I would get the CM-6 spark plugs which makes a 9" width.
The reason being is - all it takes is a minute piece of carbon to foul a 1/4-32 spark plug - so I see the CM-6 as being a more reliable option.