I read back over your post and realized that your building your oven laying down instead of up right. What's your idea behind that? What do you figure you will be coating?
As for placement of the elements in that configuration, I would plan on them in the bottom. I all depends where your putting your door, top or front.
You can find out what BTU's you will produce by looking on this web site (
http://www.heatershop.com/btu_calculator.htm ). It will ask you for "Fahrenheit temperature increase" I put in 400 and then put in your oven size and total watts. It will tell you roughly what the BTU will be.
Have a peek at mine. I used what I had laying around for the frame 2X2 square tube. Tried to make it as versatile as I could, 3X3X6. Should have built it 4X4 instead.
http://s80.photobucket.com/albums/j1...20Coat%20Oven/