I had an Interstate 1100cca in my last 1st gen (1990 W250) for eight years and NEVER had a problem starting it, including -0 temps of northern Vermont in winter. When I got my "new" truck, a 1992 W350, I immediately put an Interstate in it. That was two years ago and still no problems. You don't need two batteries if you have one good one. Also, if you do use two batteries and one goes bad, it can damage the good one unless you're using a battery isolator like the ones Ford uses on their dual battery set-up.