Good luck with all of this and maybe we can share notes.
I also am in metro Boston with a new to me 1914 balloon build. Heated with cast iron baseboards and rads w/ a Buderus mod-con boiler.
What started as desire to add AC has turned into many contractors advising we disable the hydronic heat and go with ducted hi-v heat pump w/ hydro air for both heat and cool (Unico). A recent build that did exactly this is 70 Elm in Concord MA.
One of the things I'm learning and very frustrated with is the local contractors (I've talked to a lot) are more installers than system designers. So I am now looking to hire an engineering firm to design the HVAC system. You and I should not have to perform our own heat loss calcs!