Big old cast iron radiators are GREAT for gas-fired modulating condensing boilers- FAR better than fin-tube base board for two reasons: They put out predictable amounts of heat even at sub-100F water temps, and the thermal mass of the water keeps the boiler from short-cycling when running at low, low condensing temperatures.
HELL yes you should be concerned about the -105 being oversized! While the number & sized of the radiators is an important factor in the system design, it's not how to size a boiler! It's highly unlikely that ANY 1200' house in MA that has glass in the windows would need a boiler the size of the ALP-105. Even an uninsulated house with single pane windows would usually come in under the output range of the ALP-80.
For a sanity check, what's your exact ZIP code, and how much oil do you go through in a year? If you have a mid to late winter oil bill with a "K-factor" stamped on it, what's your K-factor?
Edited to add:
The burner on any boiler has more output than a standard 40 gallon gas fired tank, and operated as a "priority zone" you get the full output for hot water heating when needed. But you still have to size the tank for the biggest tub you want to fill. First-hour ratings are all about recovery rates, but what you really care about in filling bathtubs is the first 8 minutes, not the following 52 minutes. Most folks getting satisfactory hot water performance out of a 40 gallon standalone will do fine with a 30 gallon indirect, but you have to pay closer attention to go smaller than that.
On the sizing issue- many crusty old-schooler HVAC guys will walk into a place and use a rule of thumb like 25BTU/foot for newer construction, 35BTU/foot if it's decrepit Victorian or older, and that will RELIABLY oversize it with quite a bit of margin. For a 1200' house even those overly-generous rules of thumb would still put you well under the output capacity of the ALP80.
I live in a ~2400' circa 1923 bungalow in Worcester (about a 5F colder outside design temp than yours) and you could heat TWO of my houses with an ALP80. If you're inside the Boston city limits in a 1200' house even the ALP80 is going to be on the large size, but the thermal mass of the high-volume radiators will be sufficient to keep it from short cycling, and it'll still deliver good efficiency. Odds are pretty good that your heating needs at the 99% outside design temp are about 18-22,000BTU/hr, so even on "design day" it will be cycling on and off, but with a minimum input of 16,000BTU/hr the number of burn cycles you would get with the ALP80 over a season would be measurably lower than with the ALP105 (which has a 21,000BTU/hr minimum fire.)