"The sear, which also serves as the bolt stop, is positioned in a recess under the rear of the receiver, it is held in place, and pivots on a pin through the walls of the recess. Upward tension is supplied by a small sear spring between the receiver and the front of the sear. A flat and slightly inclined cut in the bottom of the bolt body, starting near the rear end deepens toward the front of the bolt, where it ends in a hole in the bolt. As the bolt is pulled back it slides over the projection on the sear, and the bolt is stopped when it this projection contacts the edge of this hole." This if from the book The Rifleman's Rifle.
I have changed the trigger on several Model 70 Winchesters and I have also installed several Timney #401 triggers. I don't know if the Timney sear is tall enough to stop the bolt but it seems like the sear on the Timney trigger would be too fragile to be used as a bolt stop. I think if I were you I would install a new trigger spring to increase the pull. I have an article about installing a Model 70 original trigger on the Model 54.
In the 1994 Handloaders Digest there is an article by R.A. Boone where he says: The action is a highly modified Winchester Model 54. The trigger, safety, firing pin and bolt handle are modified to Model 70 style. The only outward differences between this gun and a fine pre-war Model 70 are the one piece trigger guard/floorplate assembly and a flathead screw at the rear of the left side of the receiver. This screw holds the bolt-stop in place. This was necessary since the trigger, which also performs the function of a bolt-stop on the Model 54, was replaced with a Model 70 trigger, which works only as a trigger.
I'm just guessing here because I can't see your rifle, but if there are two holes on the left side of the receiver which were used for a receiver sight, that R. A. Boone used one of these holes, probably the front one, to use a longer than normal screw which contacted the left bolt lug which stopped the bolt. Expanding on this theory, he could have put a small nut on the end of this screw on the inside in the lug race which stopped the bolt. Then a Timney #401 would work but it would be held in place only by the large sear pin of the Model 54 plus the tension of the set screw. Just guessing.