So the supply trunk you like a little tight, and you like good sized branches and returns? I suspect it won't be hard to open the holes in the trunk a little and put in bigger branches, and maybe bigger floor outlets. House would look better with new hardware on the floors too.
The branches (take-offs) should be sized according to CFM needed. The floor outlets also sized according to CFM. All take-offs should have dampers. Your house as you stated is 30x40 so as far as beenthere's comment goes I would ask how far do your supply trunk lines extend? In most cases they go almost end to end and that is what I am assuming being your furnace is directly center of your home as you already stated. Remember to base everything off the calc.'s though.