It is harder to down-size an old gas furnace than an oil furnace, but it can be made to operate more efficiently.
-- Increase the fan speed to draw more heat out of the furnace.
-- Increase the "fan-on" time before and after operation to draw more heat out of the furnace.
-- Install an electronic pilot light to do away with the continuous burning traditional pilot light.
-- Chimney heat reclaiming devices or "flue heat exchangers" are illegal in Canada on gas installations because they cool the gases down too much, forming acid inside the chimney.