"Conscribed" vs "conscripted"
As to the main point of your question, you should use 'conscript', since 'conscribe' actually has two meanings, and is not nearly as common. (This spell checker doesn't even recognize it!)
The preposition could be equally either. 'Conscripted into the army' makes just as much grammatical sense as 'conscripted for the army', although the latter puts a slightly higher emphasis on the 'army' than does the former; just because 'for' is more of a Preposition Of Interest, and draws one's attention to its object more than 'into' might.