Sako,
To answer your question regarding your XR being a good platform for the 6BR, the answer is HECK YEA!!! Awesome platform to build a 6BR.
I would highly recommend you go with a Bartlien 28", 1-8 twist, 5R .236 bore barrel. I have one on my custom 6BR and it's without a doubt the best barrel I have ever owned, hands down. If I were you, I would buy the Bartlein over the Krieger. Don't get me wrong, Krieger's are awesome, too. I just prefer the Bartlein, plus the guys at Bartlein are excellent when it comes to customer service. They not only make an excellent barrel, they care about their customers and will answer any questions you have. IMHO, Bartlein and Kriegers,Krieger has excellent customer service, too) are the best barrels out there.
All you need to do is buy a custom barrel and trigger. You already have a great acton and stock. Regarding the trigger, buy a Jewel and don't look back. Have your stock bedded and have the action trued and you will be ready.
Regarding chamber, go with a .272 no-turn neck Lapua chamber. Also, have it throated so you can shoot 87grn through 105grn bullets. I had mine throated with a .100 lead.
You are going to love the 6BR, it's an awesome cartridge. This is my first time owning one, and I can't believe I waited this long to have one built. It's freakin accurate as hell and easy to shoot. Read my post called,Bartlein Barrels, LSR and the 6BR are Unbelieveable!!!) on this exact forum. During barrel break in, by the way not much break in with the Bartlein, I shot a group in the .1's and several in the .2's. I haven't even started load development yet. I can't wait to see what it does after I do some testing. If you have any questions, feel free to private message me.