Trick:
Center main with stacks on right & left solve the 1600/800 amp rating.
IMHO, I would contact GE, and have one of their sales/engineer guys re-look at the 'design'
As to the 'custom' inquiry, there is one shop over here in NJ (UL) that can solve design problems, but you are not close to here.
One question, did you have to submit details, specs, 1-line to the local utility and municipality??
