I lowered the floor some. The plenum will have some awkwardly shaped wood in 4places to help lengthen the runners and 'bellmouth' the extensions. I'm not sure I'll be adding much of a center divider. I do not feel it needs ANY reduction in volume for any application, if anything that bridge down the center helps RUNNER LENGTH. If we could afford more delays I would split the top and locate the carbs more central to the ports. IN THIS PIC the runner roof was just roughed-in quickly AT THE PLENUM to see better the rest of the as-cast alignments.