Scored me an Offy!

Given the goals you've set the 7¼ should be fine. Mopar put 7¼s on 273 4-bbls with auto trans unless a HD axle was ordered.

One of the advantages of running the smaller axle is lighter weight. This is lighter unsprung weight which translates into better axle control by the springs and shocks. This in turn yields a more controlled ride with less tendency to hop sideways over sharp bumps.

If the 7¼ does fail then consider an 8.75 Mopar unit. The most intriguing alternative I've seen are the 8.8 and 9 inch axles from the first generation Ford Granada, Mercury Monarch, and Lincoln Versailles. Spring perch separation is 43 inches and they are within a fraction of an inch of length of the Mopar units. The Versailles even had disk brakes on the rear. The drawback is that the U-joint on the Ford does not fit the Mopar and vice versa. A good driveshaft shop ought to be able to shorten the driveshaft as well as install the necessary fixtures to allow a viable connection.