I think that the question you are really asking is, 'can a Zen Cart shop look like my other page?'
The answer to this is yes, it can. Zen is very adaptable in the way it can look.
As far as the width goes. If you want the site to be 1000px wide, which I think is a good width, then you really need to have that graphic altered so that it is 1000px wide by the designer. If you try and stretch it or whatever it will look awful which would be a shame.
So, that is step one:smile:
If you have a tight deadline - and that is a really really really really tight deadline that you are talking about - then what I suggest is that you prioritise prioritise prioritise.
What might be a good target is to get Zen Cart up and running with some products and to have inserted your (newly 1000px wide graphic) as the header. The rest of the Zen Cart page might not be stlyed yet and so won't look beautiful but most customers will understand that it is 'under development' and won't be put off. But even getting that far is going to be a challenge!
Perhaps take stevesh's advice and speak to your designer about adapting his design to work with Zen Cart.
Hope this helps,
Stormcloud