The M77 started life with an investment cast receiver in which the steel was heat treated to a pretty high level of hardness (and strength).
Being in a fire at an unknown temperature for an unknown period of time and cooled in an unknown way will leave you with a receiver that has unknown hardness and yield strength. The barrel steel wasn't originally heat treated to such high levels of hardness (and strength) as the receiver was. A cast receiver can be as good or better than a forged receiver - provided the cast receiver has close attention paid to it's heat treatment. And at the factory, Ruger does pay close attention to the heat treatment.
Then there's the bolt in the M77 (and most all bolt guns): the front nose and lugs of a bolt is also typically heat treated up to higher levels of hardness (and strength). Now the bolt is compromised as well.
The starting point might be to call Ruger and ask them whether the receiver and bolt could be heat treated successfully again. If they say "no," then that's pretty much the end of the road for the rifle unless you want to take on a huge liability. If Ruger indicates that they can re-heat treat the receiver and bolt, then it might be possible to salvage them.