Based on this thread, I think your tach wire would come through to the right of the bulkhead block.

https://board.moparts.org/ubbthreads/ubbthreads.php/topics/599026/all/70-b-body-tach-hole.html

This thread doesn't cover a 68 GTX but it does seem to indicate B bodies to be the same. 13/16 hole. Read through it carefully and you may agree.