The 40 should be good enough although I am not willing to throw my hat in the ring and say it will be!

Once running your alternator should take over the current draw of the electrical systems. I would advise you get the alternator tested to make sure it has the 90 amp reg in it and not the 60 amp that I find a few Cossies have been poked with. 60a is not strong enough with everything switched on, though I will assume you won't have things like front fog lamps or heated screens on your car? If not then electrical demand won't be as great although this could be counter balanced by additional fuel pumps etc....