Hi Joe,
No problem at all to use the 6 1/2" rims on your 280. The V8's were supplied with 6 1/2" rims to support a 205/70-14 (Except for the 6.9 which took a 215/70-14). The 6 cylinder cars had a 185/HR-14...actually an 82 aspect on a 6" rim and a 205/70-14 is the equivalent in circumference in this size.
I have fitted 6 1/2" rims to the 280 SE so I could run a 205 section tyre and as the front end is the same as a 350/450 you'll have no issue. In fact, the only issue I had, was finding some 6 1/2" rims. If it wasn't for RobertD, I think I'd still be looking!
Regards,
Carl.