Whether or not you can optimize the bullet for your gun depends upon the as-cast diameter it throws, and whether your rifle will admit a larger bullet or not. There's ways to enlarge a mould, temporarily, to see if a larger diameter has any effect. As in, aluminum tape.
My RD mould throws a bullet that measures .359 or so, with the measurement ninety degrees to the mould part line going as large as .360". You can cast of a high tin lead alloy to enlarge that dimension slightly (linotype is good for this, but it is rather hard......but the RD bullet would work pretty well even if it doesn't expand at all).
I happen to have an old Lyman wadcutter that casts to .362" and that size will fit in a fired case, but resists easy insertion of the bullet all the way down into the neck....in other words, there's some friction fit present without sizing, and this defines the maximum diameter the rifle will accept.
Since a lot, if not most 336's have really no throat at all, the idea is to fully fill the chamber neck to the extent possible, and you must check your fired brass by inserting the oversized bullet to ensure you've got release clearance in the chamber for that oversize.
In theory, anyway, a maximum diameter bullet gets a tilt free start into the rifling, and the chamber neck is therefore your throat. Heard the oft repeated advice to "slug your barrel?" Truthfully, that advice is less helpful than slugging your throat, if you have one, as it is more important to match throat size than barrel size, as the throat is almost always larger than the groove diameter of the barrel. If, like on a 336, you have no throat, then the chamber neck diameter is important to know, as it tells you clearly the maximum safe diameter of the bullets you should be shooting, and once again, this usually exceeds the barrel diameter.
The idea is this: Fit to the throat or chamber neck, not the barrel. Compared to these two things, barrel diameter is less important, and "slugging" the barrel to feel for tight and loose spots is actually more useful information than determining the exact barrel diameter.
We talk about "obturation" all the time, and truthfully, it's difficult to determine exactly when or where this occurs. If it occurs, it's often called a good thing, but in truth it's possible for it to unbalance the bullet to a slight degree. Ideally the bullet upsets as little as possible on the way out of the chamber so it maintains the balance that it had in its as cast condition, and the way to minimize upset (if it occurs at all) is to make sure the bullet doesn't have much room to upset by fitting the chamber more closely.
That's not to say a bullet that upsets somehow won't shoot very well, but it may shoot better if it fits and doesn't deform at all, or to a lesser degree.
The degree to which bullet fit is necessary depends upon what you find works well initially. If, for example, a 2100 fps RD load shoots into three inch groups at 100 yards, and you're simply sizing to .358-.359" and the bullet is cast of straight wheelweights (BHN 12) this is actually quite good. In theory the bullet is too soft, but in actual practice it oftentimes is not, and straight wheelweights may shoot quite acceptably. The benefit is the lead is of such hardness that it expands quite readily, very similar to the factory 35 Remington ammo, with perhaps a little more fracturing at the very close ranges.
The nice thing with the RD bullet is that these sort of results with the "wrong" hardness is not uncommon.
The question then becomes whether you want more accuracy than that, and at what speed. In all fairness, the accuracy potential is higher at 16 to 1800 fps than it is at full throttle hunting velocities. Accuracy potential is also greater when bullets are harder and larger in diameter.
Here's where you've got to define what you want. Bullets can be hardened and still give good hunting performance. High velocity loads can still group acceptably.
Most likely, best accuracy is lower speed, larger diameter, harder bullet.
I'm not going to recommend a powder (save maybe for the reduced loads) because any number of powders can work well in the same pressure range. The cast lead bullet don't care what's on the label; it responds to pressure and velocity. So any number of powders can work....3031, 4064, 4895, 4320 in the IMR series, a whole bunch of the Hodgdon and Ramshot powders, and others I haven't tried yet.
To save time, if hunting velocities are what you're interested in, try for the 2000 fps range with the RCBS bullet (with, for example, about 35 grains IMR 3031 or 37-78 grains 4320) and see what happens. I'd go 2000 to 2100 fps with the RD bullet and take it from there.
Both the RCBS and RD are very good designs. The RD bullet has a bit more accuracy potential due to equivalent bearing surface and less unsupported nose weight than the RCBS bullet, but the RCBS shoots very well. Certainly no slouch.
Since the RCBS bullet has more "bore riding" nose length, it will be beneficial to ensure it actually bore rides. Check the fit of the bullet nose by inserting it into the muzzle. In a perfect world the bullet will fit with a very light rifling imprint on the forward section and slight but noticeable resistance. If it is noticeably loose, experiment with aluminum tape to fatten the nose of the bullet up a bit to see if you can improve the bore ride, but you'll need a .360-.361" sizer custom made (maybe by Lee) to size the bearing surface of the fattened bullets so they may chamber. In this case you'd be filling chamber neck while ensuring bearing for the forward section of the bullet.
That usually means best potential accuracy.
First things first, though. Check to determine the distance from the end of the chamber to the start of the rifling. Most 366's don't have a throat, but to my great surprise my 2003 336 does have a throat, and a fairly long one, so I'll not make any assumptions on just how your rifle is equipped. This will help define seating depth that is functional. The front band should just kiss the rifling but upon ejection should not take undue force to remove from the chamber.