Indeed the Ruger 77 trigger is adjustable (on earlier versions), but it is not easy or intuitive. It is made unlike any other. I doubt that I can remember exactly how to adjust it without the trigger in hand.
I have a Ruger 77 project gun that has a replacement heavy barrel chambered in 6mm BR. I shot the gun in its original configuration of 243 and adjusted the original trigger at that time. That was over 10 years ago and all I remember was that it was not easy and not intuitive or like any other trigger system. You have to figure out what each adjustment does. Below is a link to the Brownells schematic of the gun:
I can tell you that #39 is a lock set screw and has to be unscrewed before you can adjust the sear engagement screw#72. As I remember it one or both are locktited in place. The actual sear engagement surface of the trigger is a fork of metal on the trigger whose engagement is adjusted by #72 screw but the lock set screw must be loosened first. Once adjusted you MAY have to readjust the overtravel adjustment screw.
The pull force is adjusted with the set screw in the trigger, which is available without removal of the receiver from the stock. I believe the overtravel is set by the set screw in the front of the trigger housing. But you'll have to verify that.
For the sear engagement it appears the trigger housing has to be in the receiver to see what the engagement really is as the sear is pinned to the receiver and the trigger is in its own removable housing. Do not adjust until only the points of the sear engagement surfaces are engaged. There has to be some overlap of flat surfaces (probably 0.010 to 0.020 inches). The sear engagement controls the creep and the safety of the trigger. Get it too little and the gun can go off from a bump.
The only way to be sure of these adjustments is to examine the trigger system and figure it out. The instruction manual for the gun only covers the pull adjustment and says to leave the other adjustments alone... probably good advice if you don't understand exactly what the other adjustment do and where they are in the assembly.
Later Ruger 77 (is it 77 MKII ??) triggers are so designed as to not be adjustable and a Timney replacement makes sense for them. Since yours has a tang safety I think it to be the original (early) version.
So in short unless you can exactly figure out the trigger system, only adjust the pull level on the bottom of the trigger (no need to remove the receiver from the stock) if your particular model has that adjustment (??).
Sorry I cannot be more detailed but the Ruger 77 trigger is not very straight forward until you understand exactly how it operates and I don't want to mislead you with a poor recollection of how to do it.