A wheel width question

[66FBCharger]

I am considering putting 275/60/15 tires on the rear of my wife's 73 'cuda. BF Goodrich recommends a 7.5"-9.5" wide wheel. I have 2 extra 15 x 7 Rallye wheels I was planning to use.
What are the disadvantages to using the 7" wide wheel vs. a 7.5" or wider wheel?

[Kern Dog]

The tire tread is usually distorted if the rim is too wide or too narrow. In the case of a wide tire on a narrow rim, the edges of the tread may appear rounded off. You are not getting the full benefit of the tire width. Not ideal but not dangerous or unsafe.

[cdr]

275 60 16 ?   or 15

[66FBCharger]

I am considering putting 275/60/16 tires on the rear of my wife's 73 'cuda. BF Goodrich recommends a 7.5"-9.5" wide wheel. I have 2 extra 15 x 7 Rallye wheels I was planning to use.
What are the disadvantages to using the 7" wide wheel vs. a 7.5" or wider wheel?

Whoops! 15".