Hello Gert;
Great project. The 8x68S is a great caliber. I used to have one barrel for it for a Mauser 66. The 8x68S adds ~100 fps to the hottest load you could ever dare with a .300 Win Mag. This puts you real close to .300 Wby country, a nice place to be with a cartridge that is not encumbered by a belt.
The only problem with it, of course, is that by the time you are done with the 8x68S built, you will have no other choice than to start building a carbon copy of the rifle in 6.5x68 ;-) A matched pair of rifles for the two Schüler 68 mm cartridges will wreck terror on any plains game of any size, any where, and truth be told, will also do nicely on cats and buffs where legal... (and elephant with solids).
One suggestion if I may: allow me to recommend that you attach the claw mount bases (front & rear) to the action with not only screws - as you have drilled and tapped for, but also with a silver solder joint. I had this done on a number of my rifles, and it is the best possible mounting system short of integral bases. This takes one variable (possible loosening of the bases screws) out of the equation in term of sights consistency.
As for scope, I would suggest that you find a used Schmidt & Bender or Zeiss rail scope that you install in the claw mount by cross drilling the rail where the claws need to be positioned. This too is virtually foolproof and will prevent any scope movement ... ever. Yep, technically a 15 year old scope will not be as bright as the latest glass, but I doubt that you will ever see a different under field conditions, and these rail/claw mounts - if well installed - NEVER go out of alignment.
Regarding 3 vs. 5 grooves, I agree with what has been said. For what it is worth, the US military went with 5 grooves for their .300 Win Mag new sniper system.
Great work. Congratulations!
PS: I have not read every post along the 11 pages, so if you have already planned on silver soldering the bases, I apologize for missing it...