mage182: The return water is going to be colder when the house is cold. The more relevent numbers will be be gotten when the house as at or near the conditioned space temp you're planning to run it. But yes, return temps below 120F are good, but short-cyling will still rob the system of efficiency.
I doubt you'll be able to swap burners to "convert" to a smaller unit.
It's hard to estimate the heat load of antique homes with uninsulated air-leaky second floors, but jadnashua is correct- it's still going to be oversized for even a fairly leaky house that size in an L.I. climate (maybe even oversized but ridiculously so for an uninsulated house in the Adirondacks.) And blowing in some cellulose and doing even moderate amounts of air-sealing would usually be a cost-effective comfort & efficiency improvement cutting the heat load (and bills) by a few 10s of percent.
David1: The guy almost certainly didn't do a heat load calc. If he did, he should be able to print out the room-by-room calclulation along with the parameters used, etc. (I'll bet he did one of those "lessee, it's mostly antique, figure 35BTUs a foot, times 2700 come out to 95K, not much margin for the 105, so mebbe we should bump it" kind of calculations. If history is any guide, the 150 would also be considerably oversized for almost any 2700' home in Leominster MA too (even half-antique.)
My 0F heat load (measured) in Worcester is about 30K, and that is in a circa 1923 2x4 stick-built bunglow with known gaps in the insulation, mostly cathedralized ceiling in 6' rafters (not much insulation there, even where insulated), antique double-hungs w/circa 1988 exterior storms for 80% of the glazed area, etc.. It's a bit over 2000' of fully conditioned space and a bit over 1500' of semi-conditioned basement (where it never drops below 65F down there after insulating the basement walls and sealing/insulating the foundation sill & rim joist) so it's pretty much "as good as" conditioned space from a heat loss point of view.
With all zones calling for heat the system is putting out ~42K (measured by delta-T x flow on the boiler loop) due to the hydro-air coil running the main zone when the radiant floors aren't keeping up, but it had no problem keeping up at -8F outdoor temps earlier this season. I literally CAN'T get more than 42K out of the heating system at the water temps & emitters I'm running (!), and that's less than half the full-fire output of the -105. (It modulates higher than that- up to ~60K out under heavy DHW load, but that's a whole other aspect of how I configured the system.) Your conditioned space may be somewhat bigger (maybe not, depending on how big your basement is and how the space is configured, but unless your windows are all leaky single-panes-no-storms your heat load would be well within the bounds of an Alpine-105, and most-likely within the bounds of the -80. (Single-panes with storms have U-value of ~0.5, so figuring 15 square feet per window the glazing factor adds less than 15K to the total at 0F with a 70F interior. Even if contractor-grade double-pane you're looking at less than 10K, lower still if better-quality.)
Your smallest zone is 60' of fin tube, which would deliver only ~15K at minimal condensing temps- about half of your mid-mod output. Your bigger zone is only ~10% bigger so the 150
WILL short-cycle on zone calls if you try to run it at condensing temps. You'll probably have to run it at ~150F or so out, and tweak the flow back to get it to run min-mod with continuous burns between calls from the thermostats, and you'd be running ~87% efficiency. With a buffer tank in series with the boiler so that it particpates in every heating system zone call to stretch out the burns to 10min+ you could run it at low temp and hit the low-mid-90s without cycling the boiler into an early demise.
In general, unless you've taken HUGE measures for reducing the heat load by weatherizing, when in doubt, go lower, not bigger on the boiler sizing. Since this was a replacement boiler, you probably could have done a pretty good whole-house heat load based on last year's gas usage and the approximate efficiency of the old boiler.
With 2 teenage girls taking endless showers you might get better mileage & efficiency out of installing drainwater heat recovery than by installing a SuperStor. Either would give you the capacity you seek:
(EFI in Westboro reps the PowerPipe version, and
will sell direct out of their WI warehouse.) If you have 5' of vertical 4" drain downstream of the shower it can pay off- more than doubling the first-hour rating of a 50 gallon tank by returning half the heat to the incoming water stream. If you later installed a SuperStor when the standalone craps out the efficieny boost would still be there if you feed the SuperStor via the heat exchanger. Bigger is always better on these (longer, bigger diameter), since the installation labor stays the same whether it's a 30% unit or a 60% unit. Payback is shorter for the bigger units if you're running the shower more than 30 minutes/day.