Hi Paolo,
Not a stupid question at all - 427 is as you say the cubic capacity of the engine in inches, but this is a 'loose' description as the actual capacity is related to the exact measurements of bore stroke etc.
I came across a number of 'auto math' calculators - some of which can be found here :
Automotive Converters and Calculators
Many seem to give slightly different results as I guess their calculations are rounded to some extent, possibly up or down.
A cubic inch is approximately 2.54x2.54x2.54 = 16.38706cc - so 427inches would be approx 6997.297cc give or take a bit.
I also came across this when 'googling' - a bit off topic, but some people have obviously got too much time on their hands...
Gloriously Glam V8 Engine Computer Case » Thoughts from the Sidelines - Technology, Gadgets & Curiosities
SAD!! (hope no-one on here has one....
)