I have built a few rifles that are working pretty well at long range and will tell you there is no rocket science to doing it. BUT, what it does take is fine attention to detail to make everything absolutely as near perfect as we can get them to get the most consistancy at long range.
I have never seen a Shilen barrel set up for a Rem 700 that was prechambered and threaded in the 338 Lapua chambering. That aside, a properly built full custom rifle
shoots so well simply because every part of that rifle is hand machined and fitted for perfect fit and alignemt. Again, not rocket science but every custom rifle
is built with one goal in mind from the time you get the componants in shop until you tighten that last receiver screw on the finished rifle, perfection!!
Now, your receiver choice is a good one and from what I hear yes they are ready to build on right out of the box.
From there though, I would have to steer you away from a pre fit Shilen. Spend some extra money and get a world class barrel, Lilja, Rock, Krieger, Lawton, Broughton, Hart. Any of these will get you world class performance if fitted properly.
Fitting properly means that when your cutting the receiver threads, you go from the barrel threading into the receiver in a 1/2 thou cut pass. Before the cut it will not fit, after the 1/2 thou cut pass it will.
That takes time and the desire to make it as quality as you can.
Then the desire to strive for perfection also comes in the tooling used to machine the barrel from live piloted center cutters to set up the barrel for machining to live piloted chambering reamers and crowning tools and the vast selection of varying bushing sizes to get that perfect bushing to bore fit, not just close enough.
These tools are expensive but the only way to do it right, I suspect the prefit shilen will not be machined with these tools and there is no way the threads will be as quality fit as they should be for extreme accuracy in a finished rifle.
The larger the chambering the more stress is applied to the rifle system when fired. The more stree, the more the flaws in the rifle will show up down range. A big rifle like a 338 Lapua need the best componants and fit to get extreme accuracy.
Its easy to build a 308 Win to shoot 1/2 moa or less. YOu need as close to perfection to get a 338 Lapua to shoot to this level of consistancy or better.
Mainly what you are missing in your project is the desire and experience to built that "perfect" rifle. No rifle is perfect but for me and many other smiths on this board, that is our goal everytime we start a rifle project.
Thats what you get for your considerable expense.
Ya ever heard the phrase "been there done that" Well, you may well get all your componants and assemble them and you may well get a fine shooting rifle but it is at least as likely you will get a rifle that will shoot no better then most factory rifles. Its a crap shoot. If it shoots great, if not, you spent all this money and will have to spend alot more to really get what you want and in the end you may well end up spending much more then you would have if you just ordered a full custom rifle from an experienced smith that knew how to build a rifle to meet your goals.