Check this recent post by Ken. He might be able to give you some helpful advice.
Latest Oven Controller Pic
The obvious problem is that it is not practical to heat an oven this size with electric. The wattage requirements would put the current draw at over several hundred amps, far exceeding the capacities of most home/shop service. From what I've seen the very large ovens like this are gas fired, not electric.
You could also post a question on the Eastwood Powder Coat forum as there are a lot of helpful guys over there that have some rather large ovens. They might at least give you some good ideas on where to start and how they supply heat for this size oven.