1 dirrect or multiple tie ins

Im currently installing new duct work in a home about 1600 sqft. there are 6 runs downstairs and six upstairs. this house was originally a 1 bedroom and 1 story. now its a 5 bedroom 2 story. 4 of the bedrooms are upstairs, those rooms are feed heat through a 10" run that shoots from the basement up to the attic then branches off to 6 seperate 6" runs each about 5' to 7' from the attic main supply. there is no insulation in the celling so the house looses heat pretty quick.
the house has a 5 ton 93% high efficiency carrier horrizontal flow furnace it is about 5 years old. It has 1 return air that is a pan return.
My question is would it be better to run 1 14" placed dirrectlly on top of the plenum wich would lay horizontally with the furnace branching to 2 12" one that feeds 2 runs down stairs about 15' from the supply and the upstairs. the other 12" feeds 4 runs 2 6" 1 about 5' from the main 1 about 5' from that run and 2 8" about 15' from main. these also have manual dampers to close off @ the 14x12x12 wye and the12x10x10 wye so the home owner can dirrect the flow to upstairs. Or would it be more efficient to to have more than on supply cut out off the plenum's sides.
Your imput is greatly appriciated cause im stuck if you have any questions just ask thanks