If you're not dead set on a Ruger M77, you can put a 3 position safety on a Rem M700 for less than you'd spend to tune up the Ruger. Go to www.brownells.com
to see a good selection of 3 position M700 safeties. If you already have a M77, and don't have the M700 yet, then it may be worth your while. I don't have a specific recommendation for a M77 'smith, but I'm sure somebody can do it for you.
I think the Ruger is a fine rifle, and have a nice one in 30-06, but in the end, for any "custom" work, you're better off going with the Remington.