A 2 3/4" loaded shell will drop right into a 2 1/2" chamber the problem is the crimp will open into the forcing cone, you said that your gunsmith says he lengthened the chamber to 2 3/4". I don't believe that there is any factory shell that will reliably drop a coyote at 40 yards with low pressure. There is some 2 3/4" #3 Buck out there but it is a pretty stout load, I would personally shy away from something like that in a fine old gun like that. Remember you have just had the gunsmith remove metal from the chamber where the pressure is the highest. I would look at handloading a nice low pressure 7/8 oz NP BB or 2's and keep the coyote shooting under 35 yards and use the rifle for anything longer. I load my short chambered 16ga at 1 oz and have no problems killing coyotes close in. Remember to aim for the head and neck with shot loads not the body.
