I concur with Dennis. Go with the Manual J calculation for proper size. Also see who went through the manual for the ductwork sizing. Proper sized unit plus improper sized ductwork=bad results. The old 1904 ductwork may not work perfectly well with the new equipment of today. It's like putting a new engine in an old junk 1955 Nash with a bad transmission, etc.
Post back with house size, number of floors and size, and list the three different recommendations specifically.