what would fit better if the front and back had +66mm offset , or if the front had +64mm offset and the back had +68mm offset? these are on a 95' z28 and 17" in front and 18" in back

 

Depends on the width of the wheels.

A 68mm offset on an 11" wide wheel wouldn't work without tubbing. It would put it about 1/2" into the inner fender, approaching the lower control arm. A 68mm offset on a 9.5" wheel would not have the backspace problem in the back (although it would be close), but would look like it was sunk way too far into the wheel well, locating the outside edge of the wheel/tire approximately where the stock 16x8's sit.

You need the specific wheel size.... e.g. 17x9, 17x9.5, etc..... in order to answer the question accurately. But in general, these huge offsets move the wheel deeper into the fender well.