Ok, but do you absolutely need to use the other outputs on the alt if it's a secondary alt? Also, I'm not trying to replace the original alt in my truck, it's only a few days old.
yes this is possible i am doing this exact thing on my truck i have a '91 c1500 and i am putting a 400a mechman alt on it as a secondary for an 18v system but the new alt is for a new body style truck with the ls based engine call mechman and they can hook you up proper you just need a single one wire turn on lead and you are golden