RidgeRooster,
For your Sendero stock, I would recommend a barrel contour of at least a #6. Just for an example, a #7 Lilja is basically the same contour as a Factory Sendero barrel but with a longer full diameter barrel shank. The #6 is slightly smaller, about a lb lighter and should still look pretty good in your stock.
Fluting a barrel of 26 to 27" will drop around 1/2 lb off the barrel if you use 6 large flutes.
The 338 barrels will certainly be lighter but not by as much as you would think. The difference really is not worth consideration in my opinion between the two.
For elk, both will work fine. The 300 will be slightly flatter shooting (depending on range you shoot) but the 338 will have a bit more authority on game. Both are completely up to the challange for elk and any realistic range.
I would agree that if you are going to go with a custom barrel, get one suitable for the heavy bullet weights, 200-220 in 30 cal and 250 to 300 in 338. For a repeating rifle, the 300 gr 338 bullets may not be what your looking for but there are alot of 250 gr pills that will perform very well and do not overlook the 225 gr Accubond in 338. Very impressive bullet for this class of round.
Get the receiver fully accurized, get rid of that factory extractor and have your stock skim bedded. Tune the factory trigger down to 2.5-3 lbs or get an aftermarket trigger if you want a lighter bullet and you will have a serious elk hammer in either caliber.
Good shooting!! Keep us posted with the results of your project.
Kirby Allen(50)