Chimney should be as far to the front as possible. Make sure there's plenty of insulation and it's sealed in well with render. As mentioned above, put a nice extension on the chimney to keep the neighbours happy. A chinaman's hat on top is ideal for when the oven is not being used and allows you to cook while it's raining - with the right wind direction, the venturi style outlets tend to funnel small amounts of water down into the oven and bring soot and crap with it.

Curing the oven after the concrete and render has set should be very slow or youll have cracks everywhere - the moisture will generally take 4 - 5 heat cycles to get to a level where you can get the oven up to temperature without damaging it.

Also consider casting some brackets or lifting lugs into the slab should you ever decide to move house and take it away with you. A well built oven is heavier than it looks (mine weighs around the 2t mark).

Good luck. Get the pics flowing!

__________________
TD06 - Oink.