Does the car have any documentation? The 3:31 rear would have had to been a special order, and even if it was rare and this car turned out to be an original Z/28, I can tell you first hand the car would be a total DOG off the line with that ratio. To me it sounds like the whole drive train got swapped out, so while there is some value to it possibly having a X77 cowl tag, a lot of its value would be in how clean the body is. Best thing you can do if there is no documentation, is post a clear picture of the cowl tag and the vin, and hopefully a pic of the stamp of the rear end housing ( passenger side tube). We can then give a better evaluation, if the cowl tag is original or re-pop. Hope this helps