I put a Timney sear and trigger in my 77 Mk II.
It took about 45 minutes to fit, but I was filing, stoning, and checking fit very slowly.
For my tastes, the Timney springs were actually too light. So, I put the factory trigger/sear spring back in with the Timney parts. With the Timney parts and Ruger spring, trigger pull is around 3.5 lbs (WAG), crisp, and clean.
I went with the Timney, because all fitting is done on the new parts - not the existing safety. So, if I want to put the factory trigger and sear back in the rifle, they drop-in with no safety concerns (other than function checking).
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."