Most cape houses have low ceilings and not a lot of wall area to work with, which might limit your mini-split/multi-split options, but maybe. (12 years ago they were probably building them with taller ceilings than those built as summer homes pre-1960 that were only later converted to year round use.)
Ductless would be by far the more efficient solution than ducted, and with better models you'd be able to heat the place even at design temp most days, at a signficant discount from heating with oil. The 99% outdoor design temps in coastal MA are all roughly +10F (Boston is +12F, Weymouth is +11), but the closer you are to the water the higher it will be.
Being pretty much surrounded by water it wouldn't surprise me if the true 99% number for Hull was +15F. A 3 ton multi-split can still put out more than 20KBTU/hr at +15F with a COP greater than 2 at that temp. If you set the ductless to 70F and the boiler's T-stat to 68F they would share the load when the load exceeds what the ductless can deliver, but it would be carrying the lion's share of the load under most conditions, and the average seasonal COP would be greater than 2.5 if you get one with a decent HSPF. When it's 30F outside the ductless has both more BTU output capacity and a COP of 3+. Well-built 2700' houses of Y2K would likely have a heat load between 28-32K @ 10F, but might be even lower if the basement/foundation is insulated and the band joist & foundation sill are insulated & sealed.
Ducted heat pumps are unlikely to beat a seasonal COP of 2.0 in this climate, which would make it comparable in cost to heating with oil, but ductless you'd beat 2.5, and would even be nosing up on 3 in Hull.
Taking the worst-case, even with 18 cent electricity and a seasonal COP of 2.5 (which would be a worst-case for you, you may even hit 3), you get (3412 x 2.5)= 8530 BTU for $0.18, or ($0.18 x 1,000,000/8530=) $21/MMBTU of heat into the house.
With 87% efficiency oil at $3.50/gallon you get (0.87 x 138,000=) 120,060 BTU for $3.50, which is ($3.50 x 1,000,000/120,060=) $29/MMBTU
Most people in MA are paying closer to 15 cents than 18 for electricity, and more than $3.50 for oil. Oil would have to drop below $2.50 to compete with ductless on heating costs a price not seen in MA in a decade. At an average COP of 3 and 15 cent electricity ductless would even be competitive with mid-efficiency natural gas heating (~$14-15/MMBTU- less than half the cost of heating with oil this past season.
There are a very few high HSPF variable refrigerant volume variable speed ducted air source heat pumps out there, but I suspect a ductless solution would be cheaper if you have the wall/ceiling area to make it work. You don't need a head in every room, just high-loss/high-gain rooms, but the floor layout can make or break it too.
Air handlers & ducts in the attic are always a bad idea, especially when (as is likely) you have a semi-conditioned basement to run them in. Putting equipment in the attic that punches holes in your ceilings, and makes it near impossible to air seal is a disaster. Any duct leakage would drive air-infiltration well beyond what natural stack effect & wind forces would too, raising the heating & cooling loads.