Since you have a heating history on the place, use (wintertime only) gas bills and the nameplate efficiency of the boiler to MEASURE the heat load on the place. It's quicker and more accurate than doing a full Manual-J based on guesswork about construction, insulation, air leakage, etc. A (relatively) quick method for running those numbers lives here
. Oversizing by no more than 1.4x will be enough to cover Polar Vortex disturbance cold snaps, yet still run at an efficient and comfortable high duty cycle at normal winter temps.
With cast iron boilers you really need to get a better handle on the radiation, and with oversized bulky cast iron rads there is a high risk of destroying a right-sized cast iron with return water too cool, causing corrosive condensation on the heat exchanger plates. But right-sizing a modulating condensing boiler turns that risk created by excessive radiation into a reward, in both lower fuel use and higher comfort if you take the time to dial in the outdoor reset curve. Your system has enough thermal mass in the radiation on each zone that it won't short-cycle even if oversized, but right sizing it will still produce marginally higher comfort, if not higher efficiency.
In many cases it will be CHEAPER to install a right sized modulating condensing boiler than a right-sized cast iron boiler, due to the cheaper venting materials, and not having to design & build protections against low return water temps.