I don't believe this can be done to your M77.Firstly the action on this model doesn't house a blade ejector,the bolt doesn't have a slot for one either,and if you were to try and do this you would be trying to alter the action from scratch.
The large extractor you refer to that is on the M77MKII is the same,I agree,but the actions work totally different.
The M77 has a plunger ejector,which works in unison with the claw extractor.The claw extractor in the M77 only holds a cartridge once it has been chambered,once it is withdrawn from the chamber,it only holds it until it clears the action and the plunger ejector kicks out the case.This system is classic 'push round feed' ,but with a large claw extractor.
The M77MKII has a blade ejector,which works in unison with the claw extractor,which in turn holds the cartridge from magazine to chamber,and then back again,where the blade ejector kicks the case off the claw extractor.This is true 'controlled round feed' ,as found on the original Mauser 98 it is copied from.
I hope this isn't too long winded!
Sorry to disappoint.Don't concern yourself too much,I have one that I made into a switch barrel in 270 and 338-06,and it works fine!