If you are clocking 56k input on that 1978 furance and it's able to keep the house warm, then you know the heat loss is LESS than 56k. If the furnace is running 60% efficiency (not uncommon for an old furnace, especially when derated to 1/2 gas input) you're at 33k output, or about what a modern 80% 40k furance can do. Manual J is great, but keep in mind it had a considerable "fudge factor" built in. I've been eyeballing the Thermalzone 45k for my own 1600sq ft house to replace the 88k 80% that the builder installed in 1999. 95% efficient, 2 stage, variable speed. Ruud/Rheem makes them and they are sold through United Refrigeration.