Do you need to have 275's...well no, but do you need a V8 in a Z?  If you're like me, I'd be pissed if I put on a set of wheels and tires and found out I could've gone bigger.   
  
ZROSSA, you could have just bought smaller tires instead of requiring all the extra time to add flares. 
  
My 275's fit just fine and I'm darn glad I bought them.  I removed the springs like Mikelly did and there is no rubbing on the rears, but I have 18's and I think they can sit farther inward than 17's due to the angle of the struts.  The only rub I had is on the front and I think I got that licked now.