larobj63, many thanks for your advice again. Upgrading is not a good solution because of limited budget. Compromised portable heaters might make more sense economically.
Only 10 feet of the main duct are exposed; the rest is between downstair's ceiling and upstair's floor. All the ducts are not exposed either. If the main duct were to upgrade to accommodate with the 60k btu, so were the branches as they are only 5" ducts and the smallest bedroom is 120 sf. It would not be a small retrofit, would it? I will ask a contractor later on about this; just wondering if it would cost a few thousands for this retrofit?