I went to the supply house to get prices on Weil McClains. As you said, he told me to measure the baseboard with fins on it, add it all up and multiply by 600. So I did and came up with 258 linear feet of finned baseboard. So I need 154,800 btus (CGA-6) using the baseboard method. The sf method comes up with 2,612 x 45 = 117,000 btu (CGA-5). My assumption is I'm using input btus x unit efficiency (approx. 83%).
You don't
NEED anything
like 600 BTU/hr per foot! That's just the
maximum amount of heat that the fin-tube can reasonably expected to deliver, thus anything beyond that has absolutely no benefit, only down side. It's a terrible way to size a boiler, since it increases the cycling & standby losses.
You also don't need anything like 45 BTU/ft^2-hr- that's literally THREE TIMES what's typical for real heat loads on that type & vintage of house in your area, and much higher than most houses in Alaska. Even if your house has no insulation what so ever your load isn't going to be that high, though it might be more than half that.
What you need is a boiler that covers your actual load with reasonable margin, and runs fewer longer more efficient burn cycles. The smallest boiler that covers your load will do that.
Tom Sawyer's
"Do a K factor gas use and size it for that load." is absolutely the right thing to do, which is what I've been saying using different terms. A K-factor would be the how many heating degree-days are covered by a single therm of gas (HDD/therm) with the existing system. From that one can derive the approximate heat load using simple napkin arithmetic. There are multiple recent threads where I've spelled out exactly how to run those calculations- it's not hard, and doesn't take a lot of time.
With a wet-finger to the wind I'm saying it's probably going to come in at about 40,000 BTU/hr. It could be as high as 55,000 BTU/hr (if the place is in rough shape) or as low as 35,000 BTU/hr (or lower if you've gone hog-wild on thermal upgrades), but there's no way in hell that it's anywhere
near 117,000 BTU/hr with the windows and doors closed.
Assuming the WAG of 40,000 BTU/hr is anywhere close to reality, with 258' of baseboard that divides out to 155 BTU/ft-hr, which means you probably have enough baseboard to run a condensing boiler at 95% efficiency all season long, even at temps cooler than the 99% outside design temp. Tom Sawyer usually isn't much of a modulating-condensing boiler fan, but his advice here is dead-on:
"If you have that much radiation available, go with a high efficiency, condensing boiler."
But if you're going to ignore that advice, at least size the dinosaur to the
actual load, as measured by fuel use, not some rule of thumb from Mars that has no bearing on Earth reality, or to the absolute maximum possible output of your existing excessive baseboard. Outside design temps in my area are 0F to +5F, and the per-square foot rule of thumb heating hacks tend to use 25 BTU/ft for houses like yours, 35 BTU/ft for 19th and early 20th century antiques with empty wall cavities. Those 25 & 35 BTU rules reliably oversize by at least 1.5x, and often more than 2x. The customer never gets cold, but they're not doing them any favors- they're buying too much boiler, and end up less comfortable for it.