Darren, I've gone for 40" internal also, with the widest point where the seat goes (further back than the plans suggest). And I've made it 'square' (ie. sides at 90deg to floor). This means that it was VERY easy to get the top longeron flat, and there's plenty of room inside. There has been some discussion about the merits of vertical sides, it comes down to cosmetics. I'll try to update my website this weekend so you can see how mine looks so far...
Cheers, John -- John Bavington Secretary SAAA Chapter 20 P.O.Box 759 Sunbury 3429. http://www.saaa20.org http://au.geocities.com/johnbavington