Sorry but there is a lot of ignorance in this post, no matter how much experience you say you have.I have to admit, your intentions seem good, but I'm not certain you've ever shot an elk, or considering the more likely scenarios.
1. Elk aren't that hard to kill. A kudu is, but any well placed shot with a sectionally dense 140gr bullet with adequate energy will do the job.
- No one wants to hear the "I shot..." story, but like ballistics on 7mm-08 and was successful with difficult shots on kudu, zebra (which are really hard to kill) and a bushbuck with one shot 120gr Barnes copper solids at ranges between 247 and 392 meters. The 392 meter shot was a kudu in brush.
2. Growing up in elk country in NM, I can't remember anyone taking a shot beyond 250 yards. Maybe we we taught not shoot at an animal that you'd likely wound and find carcass parts the following spring
Where I'm going with this is that a 1000yd shoot is unethical - period.
400 yards is about the end of practical. 500 if your really good and on flat terrain.
So, if you want an 18" barrel, the gold standard 308 Winchester works well with short barrel lengths and easily mastered.
The trick to distance shooting is practice and loads, not something a normal guy can't shoot more than 10 times without quivering.
If you're intent on pushing the envelope, I hope you're willing to live with failure.
A short barreled rifle for elk in timber is a smart move. Why else would have cartridges like 348 Winchester been invented. But they were intended for use with open sights.
For the sake of the animal, practice at 200yds and understand what steel sounds like at 500, and remember your more likely to kick a big bull out at 40 yards than line up on one at a 1000.
Sorry this isn't what you want to hear, but you shoot a nice bull with a guilted 338 Lapua at 80 yds, and you probably won't recover the animal unless you're lucky and catch a major vessel.
Bullet will go straight through with no expansion and walk away wondering what kind of wasp just stung it.
Good luck with your hunt and build.
EM
Another one for the ignore list, and after one post...that's saying something.