.740-.729= .011. Had it been just .002 less it would not have needed reproofing. Any alteration greater than .010" requires reproof. Also recall that .011/2= .0055 wall removal. So if the walls were .035 to begin with they are now .0295. You do not know what they started out as so the need to measure is very much the only real way to know. Also that proof measurement is taken at 9" so at that point it has had .011 removed. You do not know if more has been removed elsewhere. Again measure the wall thickness for the barrels from end to end.
.740 wold not bother me in the least, if the walls are still .025" or greater. I would get more concerned if the walls are .020", concerned about possible future dents as much as anything. Again I want to know where the thinnest area is and decide then. At 20+" I would still shoot it with reasonable loads, but that is just me. And I have put a lot of worry into the math, the measurements and what "reasonable" loads are, which is as much a personal thing as anything. I shoot a 20 bore Pape that has a barrel with .017" wall, but that is 27" on a set of 30" barrels. Still I keep pressures down low and am careful about any actions which could dent it. It is a lovely hammer gun and a nasty dent might be the end of shooting her.