You've done excellent homework so far. And yes, a throat slug should be swaged and sent with your mold order.
Ordering an LCFN is the best solution for your rifle, as it has the same meplat diameter as the LFN, but a longer and more streamlined ogive. Also, don't get hung up too hard on holding weight at exactly 300 grains, nor rethroat the gun to be able to get that weight cast bullet. Rarely will cast and jacketed shoot to the same point of aim, so a bit less weight to insure good performance won't matter. The sights will probably have to be set for the load being used anyhow. Or, if both are carried afield, use your cast strictly for short range work, where slight sight errors won't matter, and jacketed for longer ranges. I have never experianced a change in POI when switching from cast to jacketed (or jacketed to cast), due to lube traces left in the bore (or lack of same), but those lube traces, if using LBT lube, do totally eliminate jacketed fouling for several shots. One costomer told me that he always popped one cast load (with LBT lube) through after ever 10 jacketed, and never got jacketed fouling.
Every gun is an individual, so you'll have to work with what your gun does once you get a mold and have loads worked up. However, I've measured thousands to throat slugs and found that the heavy game magnums tend to have oversize throats and groove diameters. This to insure function reguardless of extreem conditions when facing game that kills back. Also, they all operate at high pressure (60,000+) which means you'll probably have to keep your cast loads a bit below maximum to good and consistent performance in all weather.
Most bolt rifles I've worked with throw jacketed and cast into almost the same group, with the only important difference being trajectory due to speed differences. In other words, you'll problably be able to hunt from a stand where distence is known, with your cast load, then perhaps switch to jacketed when still hunting if long ranges are expected, or just wait till a long shot presents itself before chambering a jacked round.
You want to shoot jacketed, and the gun works well with them as is. Rethroating is expensive, and if you don't order a custom throating reamer can often be detrimental to the guns performance. This because SAMI specs do not appear to be a concern to manufactures of either guns or reamers, which means that just having a gunsmith lenghten the throat with a throater he has on hand is ofen wrong.
