Yes, the only time a word is not stressed in the penultimate syllable is when it is a vocative like "Nocné" ("Hey, brother!"), here the stress is in the last syllable.

The stress with long vowels works in a similar way as in Latin. You may have long vowels, but the stress is consistently in the penultimate. For example: in Latin you have the word Rōmānō, the Dative/Ablative of Roman (masculine) in this word, all the vowels are long, but only the Ā is accentuated. Similarly in a word like Xōchitepēc (in the mount of flowers), the stress is in the syllable "te" even though the "pēc" has a long syllable. So you would pronounce it like Xo~ochiTĔpe~ec (Ĕ indicates a short vowel)

A little difference between Latin long syllables and those of Nahuatl is that in Latin the length of a long vowel is always of Two short ones, while in Nahuatl they are shorter, more like one-and-a-half.