You'd certainly be OK for 600 yd BR (I'm currently shooting a 6BR at 600 and 1000) but you might be giving away a bit of velocity at 1000 yds. Why not just open the boltface a few thou and shoot 6BR or Dasher? Easy job for a 'smith. You'd be wanting a 28 or 30 inch barrel.
Bought a Vostok a few years ago for £100 - hoping to rebuild it into a 22RF benchgun. The chunky action looks good but the set triggers were not so good. I still have action and cutdown barrel if you need spares.
6x47 Lapua - I'm assuming you run this calibre? I'm shooting mine on RS60 for 1000 yd benchrest but none left so I'm looking to switch to Vhitavuori - so 150 or 160 with a 105gn bullet.
Never shot PRS but always thought the challenge was targets at unknown distances - difficult to run on a formal range with fixed firing-points and bullet-catchers - so how was this achieved at Bisley?