You either need to step up the fan speed (and probably will not be able to make it spin fast enough to move air through the undersized ducts), install a higher pressure fan and motor to make the air pressure higher to move the quantity of air required through the undersized ducts, or increase the main to 8 by 24 and leave the new extension the same size. Check the air flow quantity by taking a reading of the furnace exit temperature, making sure it does not exceed 120F.
Considering that a 100,000 btuh furnace can heat a well-insulated 4400 square foot home, you could change the furnace down to what you actually need by a doing a heat loss estimate after insulating the home to modern standards and leave the ductwork as is. The furnace you have now is just about right for a house your size is now if it was insulated as poorly as a house built in 1960.
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