Ask any of them if they did a manual-j heat load analysis for your house. The vast majority of guys just do a quick WAG on it, and almost always oversize the thing, often by a factor of as much as 5x. This is a recipe for disaster.
Without knowing what size units they are proposing, and whether they actually did any sort of valid heat load, those numbers are almost entirely useless. If you read some of the other threads here, you'll get an idea what's involved. If I was to give it a WAG, I'd guess you probably could get by with pretty much the smallest boiler you can get, which is in the area of 50KBTU or so. A mod-con that can modulate down to a small number most of the time would be able to meet those efficiency numbers for at least part of the year. One that is oversized will take a MAJOR efficiency hit, and not provide the comfort you want.
The LAST thing you want is to copy the size of what you have, since in 1957, they were almost always way oversized. Nor, do you want them to measure how much baseboard heater you have and say you need such and such a boiler. You also cannot just say you need x BTU per sq foot. NONE of those are a valid means of sizing your boiler! Nor is getting a bigger one to heat the water tank.