I think you have a very good looking build there. Someone other than I will have to comment on the durability of a Merc Alpha drive. The torque pull sure should help holeshooting it onto plane, especially if you strap on a couple skiiers or a tube. You may also find you can pull several inches more prop and get better mileage (if you can keep your hand (throttle) out of it).
Usually in my simulations on DD, I run High-perf Manifold w/ mufflers, but I am usually hooking a big block up to log-manifolds on jet. I think that may still be more accurate, unless you are running thru-hub. Then, the stock may be more accurate.
I do know folks who have put the cutout systems on a small-block, and gained nothing so the hub-exhaust isn't to bad on the small motors.
A "marine" block is the same cast iron one in a car, usually just brass freeze plugs. The build parts inside depend entirely on how hard you are trying to work the engine. My 454 is making 400HP (loafing, i know), spinning a cast crank, and the engine builders here have no concerns for me unless I want to spin it up beyond 6-large (which I do not), even to 600 HP. It's a 2-bolt as well.
This 300 HP 350, if built with care, toleranced properly (IMO, factory stock tolerances, ABOVE THE MID-RANGE of each tolerance (ie, slightly "loose")), a cast crank if you have one should be fine, stock rods should be fine. Cheap insurance on the rods would be a set of ARP rod-bolts and a re-size on them. Forged pistons are not a must (my cast ones been running 28 years, even sipping 75HP of Nitrous), but, since a set of Speed-pro Powerforged's will set you back under $400, if you have the $$, and need pistons, why not upgrade for nearly nothing. A set of casts will cost likely over $300.
IF the boat sits in the lake, engine ful of water, all summer, you will have galvanic corosion problems with an Al intake. If it only sits all weekend, or maybe a week on a big holiday, then comes out to the trailer & sits dry 98% of it's life, you will have no trouble.
Salt water is a different issue.
Rinse the snot out of it, and rinse it some more, then rinse it again. There are helpful products out there, others must fill in here as I do not make it to oceans with Lowrider.
IMO, for 95% of the uses, the stock oiling system is plenty up to snuff, especially for a moderate build level. Put in a windage tray, a oversize pan is not a bad thing, but I ran 28 years so far on a stock-sized one (a larger one is in planning, but will be a while), stock pump too. As long as you get (and can maintain) 10psi per 1,000 RPM, you are OK. A long hard run will heat the iol, thinning it. If it is severe enough, a cooler will be needed. Depends a lot on how you drive. I've not been able to run more than 3 or 4 miles, flat out, and no issues, but I am also pushing my engine harder as I update things, and I am watching.
Clean out any flashing (casting excess) from the block drains before you have the block cleaned/clean it, this helps keep drain-back from being interfered with.