On my '67 Belvedere, a 255-60x15 on a 15x7" with 4.25" BS was the best fit without issues.
A 275 will go but for me was wasn't worth the occasional rubbing. Others have done it and say they have had no problems, I've done it and I'll just say I won't do it again. There's a reason is a 275 tire on a 7" wheel is not recommended, it should be on a 8".
If you do run a 275, let the air out to make for an easier squeeze into the wheelwell.