Ok, at last, we have a gas bill. The February number from degreedays.net for our weather station is 1306, and according to the gas bill, we used 414.49 therms that month (BTU factor is 1.028). The house was unoccupied most of that time and set to somewhere in the 68-70 degree range (I kept it warm to get a realistic gas bill). One hiccup: some workmen turned off the heat one day and forgot to turn it back on, and we had a frozen/burst pipe, so for about 12 hours the heat was off entirely and for about 12 hours it was cranked to maximum. The boiler is 160/128. Do I have all the info I need to make a calculation?
So, 128/160= 80% steady-state efficiency. If they billed in therms and not CCF there's no need to convert fo the BTU factor. 0.8 x 414.49= 332 therms of heat delivered into the heating system plumbing over 1306 HDD. That's 332/1306= .254 therms/HDD, or 25,400BTU/HDD.
In a 24 hour day thats 25,400/24= 1058 BTU/degree=hour.
Assuming an outside design temp of 0F and a balance point base temp of +65F (the HDD base presumption) that's 65F heating degrees...
...which implies a design heat load of 65F x 1058 BTU/F-hr= 68,770 BTU/hr.
For a 2500' house that's 27.5 BTU/hr per square foot of conditioned space, which is on the high side for a house that size.
An uninsulated but heated basement is probably worth on the order of 15,000BTU/hr, (give or take 5000 BTU/hr depending on the particulars)- insulating the walls to R10 would reduce that to 1,500-2000BTU/hr, so knock at least 12K off that 69K number and you're at 57K (probably lower) bringing the BTU/ft down to 23, but that's still a pretty high number for a 2500' house, which implies you may have gaps in the insulation elsewhere &/or high air infiltration rates.
The 57K is just barely over the DOE output of a Solo 60, the house was not being occupied, AND the boiler is probably not delivering 80% efficiency after 50 years of service. If we use 75% (which is still a bit optimistic) that 69K number is reduces to 69K x 75/80= 64.5K, and if you whack 12K off that number for insulating the basement you're at 52.5K, which IS within the output of the Solo 60. If there are going to be a couple of live humans there sleeping cut that to 52K, and with typical 24/7 plug loads you'd knock off another 0.5-1K from there, but even 50K is a pretty high number for a 2500' house with an insulated foundation- you either have a very unusual house shape or a lot of heat leaks.
A full Manual-J would be useful for estimating where those leaks might be, but it might be worth doing a blower-door & infra-red imaging assessment to spot & rectify the most egregious leaks even before insulating the basement. Electrical or plumbing or flue chases that run from the basement to the attic can move HUGE amounts of air into & out of the house even without the house feeling super-drafty. The flue of the atmospheric drafted antique boiler can itself impart a significant heat load from depressurizing the basement, drawing in air even when the burners aren't firing. Blower doors & IR imaging can find even the less-obvious thermal bypass culprits.
Then there's the issue that they only estimated the meter reading, more often than not with a thumb in the scale favoring THEM....
The manual-J came in at 51K even with the extra 5K+ of window losses due to ignoring the actual U-factor of your windows (11,610 BTU/hr x 0.30/0.57= 6111 BTU/hr, which is which puts it at ~45K. The Manual-J also accounted for 2987' of conditioned space, not 2500'. That's a ratio of ~45,000/~3000'= 15 BTU/hr per square foot of space, which IS in the credible range, but probably also on the high side of reality, depending on where else he might have underestimated the envelope performance.