Run Flat Question

A while ago I bought a used pair of BFG 275/40 R18 Run Flats (standard on vettes). I went to have them mounted and the guy said the couldn't put them on without possibly damaging the rim. I have 18x8.5 so shouldn't it work or is the harden sidewall really a problem?

I mounted Nitto 275 40 417 drag radials on a 17x8 rim on my old Mustang, so I guess I'm wondering whats wrong.
 
With the relative number of cars that come with run-flats (a fair number, all makes considered) where do folks go then to get them mounted?

Some shop has to have nice machinery around to get it done. So what if it costs a little more for the mounting - it is worth it.

I am not agreeing or disagreeing with anyone - just thinking aloud. It just seem that every decent sized town has to have at least one place to get it done (even if a dealer was where I had to go, I would do that as long as it is not rediculous).

Then prepare for the extra road noise. :D