Frank, I agree that 4" is about the max you could go.  If you go to a wheel dealer, they usually will try one on your car to check for proper spacing and clearance.  I did that when I bought my wheels.  If you put 17X8's on the front, I wouldn't go over a 235 or 245 tire as it may rub on hard turns.  Good luck on your selections.

  I have 17 x 9.5 up front and 17 x 11 in the Rear on my 80".  I'm using C-4 Wheels and therefore I have Spacers as well.  Up front I Shine up the Frame a Bit on Locked Turns but not Bad. I'm Using 255 Tires. The same Tire and Wheel will go on the Back with Stock Trailing Arms and Spring. If You want a 11" Rim then things get Complicated Fast. Offset Trailing Arms, Moving the Parking Brake Cable, Narrowing or eleminating the Swaybar etc.....Oh I have 315's out Back. Richard.......

I took a measurement and I think I can get a 17x10 in the rear  4.5 offset with no rubbing. I have a 80 vette. Say banks I would like to see a picture of your car.

Update TMTNET2007-05-05 12:57:47

Jeff - thanks for posting an update. I've been following this thread with great interest.

I have a stupid question. I might be needing tires before the carson trip.  would the 17x8 with 4 in backspacing fit on the 70 vette?  Also would any wheel that was 17x8 w/4in bs work (don't really want what every one else has)
 thanks believe it or not i have never purchased a set of nice tires and wheels, what a lacky
Larry