Since you have a heating history on the place,
start with a fuel-use based heat load calculation, THEN move on to measuring up the radiation, broken down by heating zone. When you have both you can determine the minimum sizing of the boiler necessary to heat the place, and the water temperature at which the radiation can deliver design-day heat. The answers may steer you one way or another on whether or not a
modulating condensing boiler would be an appropriate choice. (Some are simpler than others.)
If you're stuck on the notion of a dumb as a box o' rocks cast iron boiler, it's still important to size it for the heat load, not the maximum output that the radiation can deliver. Most boilers installed in the 1950s were more than 2x oversized for the heat load, and houses built in the 1950s have had significant insulation/air-sealing/window upgrades, resulting in heating systems 3x or more oversized for the load. While some amount of oversizing is fine (ASHRAE recommends 1.4x the 99% heat load, which is enough to manage even Polar Vortex events), AFUE is tested at a presumptive 1.7x oversize factor. Above that point the as-used efficiency starts to fall off, slowly at first, but by 3x oversizing and up it's a 10% or greater hit unless you start adding "smarts" to it, such as heat purging boiler controls, etc.
