In theory, the strength of the toggle-link action depends on the backthrust on the bolt. Properly fitted, the ends of the links should bear on the cutouts on the bolt and receiver. The force is the pressure times the area of the cartridge head. The force is linear between the ends of the links. Some guns may not have the ends of the links bearing on the cutouts, in which case the force is on the pivot pins, which are in shear. In that case (NOT correctly made), the shear strength of the pins would constitute the weakness. If there is any sign of the pins bending, then the fit is incorrect and the pins should be replaced with stronger ones (using hardened drill rod, for example).
I haven't compared the sizes of links between Winchester '73's and '76's, but the size of the heads between .44-40 and .45-60 or .45-75 is different, so the '76 will have more backthrust. The best thing when using smokeless powder is to use the slowest powder that will burn most efficiently and keep the MV the same as the original BP loads. IMR4198 is probably better than 3031, but the latter will probably do, although, as reported, there will be a lot of unburned or partially-burned particles.