I have a Timney replacement trigger on my M77 MKII and I installed it myself. I would recommend having a gunsmith install it though. Mine came out ok and it is safe however I hunt with the factory trigger and the Timney spring because the Timney trigger and spring is very light, too light for me to feel 100% safe when I'm hunting. I do like shooting off the bench with the light trigger though.

You can also buy a factory replacement trigger from Ruger and have a smith work it for you for about the same cost as the Timney. It's on the Ruger web-site under replacement parts catalog. The Timney cost about $85 from Midway.