Although not recommended, a 275 will work on a 7" rim, the previous owner of my car had these 275/50's on this 72 for 8 yrs and I will be replacing them with 275/60's on 8" rims. I've had to postpone doing it because of funds, but until I do, I'll keep running them. Heck I have a Dodge truck with 295's on 7" rim's and those tire have had 60,000 miles put on them.
Here's a pic of my 72 from my post on tires.
http://chevelleforum.org/showthread.php?t=2567