The fact that your design condition heat load is ~ 37.5K doesn't mean that ~50K boilers are "too big".
The passive solar gain during design conditions is pretty close to zero, since outdoor design temps tend to occur almost exclusively in the pre-dawn hours.
The fact that these boilers will modulated down to ~10K, means that in a well designed system they would track load very well during most of the heating season, and would not lose much in terms of efficiency.
If your radiation requires 150F+ water temps to deliver the 37.5KBTU of heat to the house the combustion efficiency at typical delta-Ts will be no better than 86%, and the 50K boiler would only be capable of delivering (0.86 x 50K=) 43K of heat in to the system at full fire for a ~38K load a bit more than 10% of margin, which is OK but definitely NOT overkill, and NOT too big.
Even if you're running slab radiant with 110F water on design day you're still in the low-90s for ~45K of output, call it 15% of margin, still not overkill.
Even if you assume your heat loss calc tool is padding it by 15% and the true design condition heat load is 32K you still won't lose anything to efficiency with a min-fire of 10K, less than 1/3 of peak load. At 45K of output at design condition it's only 1.5x "oversized" and would meet it's AFUE numbers even if controlled in a bang/bang on/off fashion at max fire.
For a mod-con boiler to be considered "too big" is if it's minimum output is over half the true design condition heat load, but even those systems can be designed to run efficiently, even if you have no use for it's higher output capability. The fact that the min-fire output is well under half your design condition load makes these boiler much closer to right-sized than "too-big". The max fire number is of lesser importance than the min-fire number. Even were a 40K boiler available, it might not be advisable to leave yourself that thin a margin, if it's buying you nothing in efficiency. AFUE numbers assume 1.6x oversizing, and are still pretty close to the steady-state 100% duty cycle efficiency of the boiler. A ~50K (45K output) boiler for a ~38K load is about as good a fit as you could possibly get!
Residential application that would require upsizing the boiler above the space heating load to be able to deliver the DHW load are pretty rare. Even at design condition most homes have enough thermal mass and insulation to not lose much temp during stored hot water recovery burns. Peak loads for DHW are often 2-5x the peak space heating load, but they're very short in duration, and easily handled by the stored thermal capacity in a tank.
At tiny space heating loads under 30K it's simpler and cheaper to forgo modulation & load tracking entirely, and take a bang-bang burner buffered approach, such as a condensing hot water heater. Even a ~75KBTU/hr Vertex will run with great thermal efficiency and would have burner to spare for a 25K heat load on top of typical DHW loads. Standard gas fired hot water heaters do just fine with an 80% combustion efficiency ~35K burner (~28K out), using the mass of the stored water to distribute the instantaneous very-high BTU/hr loads of tub fills or showers over a longer burn than the shower or fill time. No more than ~1% of the time is your heat load going to be the calculated design condition heat load- the rest of the time it's warmer outside and during daylight hours you'll have solar gain (even on dark and cloudy days the solar gain is greater than zero) picking up some (or all) of the load.
Even at 1gpm you don't have to raise the system pressure much to avoid flash steam on water-tube boilers if you're pumping toward the boiler. And it's pretty obvious when you're "there". At lower pressure & flows they sizzle audibly from the steam-boil, and at REALLY low pressure & flow they'll even pop & bang. Most hydronic systems for 1 & 2 story houses are run in the ~12psi range (measured static, not when pumping). I'm running mine at under 2gpm with a static pressure of ~18psi and hear nary a peep from it. The components for hydronic systems are almost universally good to at least 30psi, which is a typical pressure-relief valve setting. Pumping like crazy with a monster pump to treat the sizzle & bang flash steam issue since it's (usually) easily handled by modest pressure increase. I put a Taco 3-speed on mine but I run it at it's lowest speed (~70watts). I could have done fine with a -007 or similar, but I wanted to be able to monkey with it a bit in case I needed more DHW capacity.)
Water tube boilers aren't nearly as difficult to design around some people seem to think, but they may seem pretty exotic compared to high-mass low-head cast iron boilers, and from a drop-in replacement point of view it's probably easier to put in a lower-head mod-con. I have no direct knowledge of any peculiarities to the Lochinvar WB- series, but while they may have an order of magnitude higher head than some other boilers, they're within the range of other water-tube boilers at similar output.