Yeah but was it born with those?
that 73 is now 50 years old, a lot couldda happened since then.
If the 340 is original, my guess is that the car is set up with the 4.5BC : but it's just a guess.
Notta chance would I run 275/60s on 7s on the street. Rule of thumb, back in the day, was to run a rim that is at least as wide as the tread. But with radials, that's a bit shy. 275s need a minimum 8.5 to survive on the street.
If you install them on 7's they will just burn off the centers. If you reduce the tire pressure until they wear a lil better, you could be down to in the range of 20/24psi. Have you ever run 15x28" tall tires at 20/24 psi on the street With a jacked up azzend?
Warning;
slow down for the turns.....
IMO that's a bad idea.
Jack up the car, remove a rear wheel, set a square against the drum tight to it, then measure from the drum to whatever is closest to it, in both lateral directions. Usually the outer wheelhouse lip will be closest, but it could be the spring.Then you do a lil math. If it's really close, then you better also check the other side; the axle could have dropped in, NOT centered, right from the factory.
Another option is to mount any 15" rim, measure it up, then correct for the known backspace of the checking rim, then order what the math says will work, commensurate with the chosen tire width; ie don't try to put a 275/60-15 on a 7"wheel, lol.
I for one, won't commit to saying anything will or will not fit, cuz if it doesn't, It's you who pays the price; and I will be less popular with you than if I just don't commit, lol..