We're putting in a 600 amp panel and the contractor is re-using 3-4/0 MC cables for the feeder but the main breaker lugs only accepts two conductors per phase. Thinking of using a distribution block with the 3-4/0 in and 2-350's out. Can this block be mounting in the panel inclosure, which has ample space? If not, what would be the minimum size j-box? Thanks.