Do you have the correct L/H exhaust manifold (casting #3892683) for a '68 Z28?
Also, see the '68 AIM for the following...
- UPC 6, Sheet A4 = lower alternator mounting bracket instructions (make sure you have all the washers and spacers in the right position)
- UPC Z28, Sheet A3 = upper alternator mounting brackets (braces) instructions (make sure the alternator braces are mounted in front of the alternator's threaded boss)
As far as the alternator itself, it's physically the same as the standard '68 37 amp unit. The only difference is the stamped part number (1100814) and the deep groove pulley.
Finally, are you using the correct alternator drive belt?
Below are photos of a properly mounted '68 Z28 alternator...