APPLES vs. ORANGES
First, I wouldn't consider the Ruger .375. In 5yrs It'll be a footnote in reference books.
Having been to Alaska (yeah, a total of 11 days, but I learned a lot. And my older brother spent 5yrs in Anchorage, and my nephew (his son) spent 3yrs at Fort Greely (near Fairbanks).
You can find .375 H&H ammo in the Walmart at Talkeetna, and Hardware store in Skagway (saw it myself!!!) Not to mention most anywhere else ammo is sold. Try that with the .375Ruger.......
Having shot several .375H&H's and owning several .45/70's, I can say that they're totally different animals. (one a medium bore and fast, the other big bore, and not so fast.....) Like a line-backer vs. an offensive lineman.
I would suggest that if you're going to spend time in AK, take one of each.
I find that I prefer my M1895GG with a Williams Guide reciever sight and fire-sight front, sighted in for my "usual" load, and the Folding rear sight folded down, but sighted in for the "HEAVY" loads (ususal is an RCBS .459" 300gr FNGC which casts to 318gr with aircooled w/w and lubed with SPG. This is loaded over 30.0gr of #2400 for 1650fps. It'll drop anything alive, but dosen't have the extra margin as a "stopper" for the great bears (brown/grizzly). The receiver sight can quickly and easily be re-set for the heavy load by referencing the folding rear sight.
The heavy load is the Lee .459" 405gr HB cast from w/w with 3-5% 95/5 lead-free solder added (tin-antimony) and heat-treated to about 25-27bhn, and then loaded over 50.0gr of either IMR4064, Varget, or RL15 (not a significant difference between them). The velocity is about 1,900fps from the non-ported 18.5"bbl. Accuracy with either is 1.5-2" at 100yds (5-shots).
Inside 100yds there won't be any arguable difference in "stopping" ability. However, the Marlin will more likely be in your hands and "ready" when something "happens".(shorter and MUCH lighter). Also, from extensive experience shooting deer and pigs, I can attest that subsequent follow-up shots are much faster with a lever-action than the b/a. (I've got 5 L/a cf's, and over a dozen b/a's and I've shot hundreds of head of big-game. (-retired game and fish officer, PPC competitor {took 2nd place in shotgun match in 2002, second highest score ever fired- 550-5x -possible is 550-10x, nat. record is 550-6x shot two relays after my score. Perfect score hasn't been shot since (even by me.......)} I used a Remington M870 with factory open sights (10rds are slugs, 5@ 50yds) and required 1oz slugs and 9pellet buckshot loads). The 5rds at 25yds have to be fired in 10sec, starting with the gun butt under the arm/shoulder. 15yd slugs, 5shots in 5sec., 4 different targets !!! So, you do have to shoot quick, and accurately !!!) But, we're not discussing shotguns.................
I know of several guides that use the guide-guns as backups. One has his GG, and another assistant with a 10ga Browning p/a loaded with 000-buck when tracking/approaching a wounded bear.... Sometimes though, it's the GG and a .375 or .458wm however, or the latter two and the 10ga. Whatever they have that day....... Read Scoville's (Wolf publishing) for his articles on hunting with Ed Stevenson in AK. Ed is a big lever-action fan.
The .350 is a good gun/cart. I would suggest a Remington Custom shop Mod-7 Alaskan (synthetic/stainless), as I believe that the 673 has been discontinued, and .350mag ammo is very rare, even from online sources. (I've wanted one in 6.5 BADLY, and let a .350 slip by me....)
I've got a Marlin .338ME, but the jury hasn't even been convened on this cartridge. My reloading/shooting efforts suggest a near clone of the likewise rare .358win. My personal choice of a second gun will be my MkX Mauser in .338/06. It has nearly the punch of the .375 (2,550fps w/250gr bullet) and instead of the 3+1 of the H&Hmag, It'll hold 5+1. In case the bear/moose just refuses to die. I've got a Leupold VariX-II 3x-9x 50mm w/heavy duplex reticle. Also, barrel wears a set of Williams guide open sights sighted to 100yds with 250gr load. Base and rigs are Leupold turn-in. Gun wears a Hogue Stock, with LOP reduced to 12-3/4" for rapid shouldering w/cold-wet weather clothes on. Besides this, I took my first deer with this rifle in it's previous life as a factory MkX Classic in .30/06 in 1976. A lot of sentimental attachment. It's ironic that the friend who owned the property was hunting with me at the time, was carrying a Ruger #3 in .45/70....
I'd suggest that you try to run down a Savage Weather-Warrior Stainless .375H&H guide gun. They make such an animal as I saw one in the Walmart in Talkeetna, AK in early June of '09. I had the exceptionally knowledgeable clerk (in his mid '50s) take it from the case for me to look at. It had Guide-Gun laser etched on the bolt. Walmar asking price was $678.00. Haven't found anyother references in print or on-line to this gun. It may be a Walmart specific gun. They also had at least 5 different loads/makes for the .375H&H on the shelf, including the Federal Premium-Partition, and CapeShok, Winchester silvertip, and Remington Corlokts mostly in 270gr. I did see a 300gr load but can't remember what it was. This gun WILL require the stock be shortened for rapid shouldering by anyone under 6'2 however...... 13-5/8" LOP as-is, is my guess.
So, it's really a matter of choice and where you'll be hunting. If on the Kenai Pen., I'd take the .375h&h (tide flats-open shots) with a removeable low power scope. If in dense coastal lower AK, I'd choose the .45/70. Or preferably, both. AND A 12ga pump shotgun.