RE: Help me build a rifle
First of all, do you know what cartridgethis action was originally chambered for? If it was a cartridge based on the .30-06 case then bumping up to a magnum will require altering the bolt face since the head diameter of a magnum cartridge is larger than that of the .30-06 family of cartidges. The bolt facething is not a huge deal, but you may also run into having to do other alterations to the action and magazine if you want to go to a cartridge the action was not originally designed for. So think about that. Personally, I'd stay with a cartridge within the same family as the original sothat alterations to the action would not be necessary,but that's just me.
BTW, I had my Sako 75 rebarreled last year by Hill CountryRifles in Texas. They put a Lilja on it and it shoots great.It was originally a .300 Wby, but the new barrel is chamberedin .300 Win. Since both are based on the same parent case, no alterations to the action or bolt were necessary. I'm very pleased with the way things turned out.