Preface: I'm not an HVAC pro - just a guy in MN maintaining my house.
Similar situation here except my house was moved onto a new foundation in the fifties and the giant boiler replaced with then current technology. I subsequently replaced the fifties era boiler with a Slant-Fin about fifteen years ago. Not super high efficiency, but I was more interested in longevity because at the time I personally knew of a spate of heat exchanger failures on newer 95% boilers and furnaces. I chose cast iron knowing I was giving up 5 or 8% eff. but I still dropped my heating bill by stepping up 40 or 50 years and getting rid of the standing pilot and open chimney. Absolutely happy with my choice.
BTW my wife LOVES the big cast iron radiators - apparently she doesn't whack her knees on them as often as I do...
Joe