'Spread' is not the ideal word to use here, because 'spread' suggests movement of some kind. Populations, fires, wars, fires and diseases can spread across a country, for example, but it would be unusual to use this word for a mountain.
'Stretch' or 'extend' would be a more likely verb. Also, is it perhaps a range of mountains, rather than a single mountain? For example, the XX mountains extend east to west across the country.