In 1966 there should have been only one version of the C4, but there are later C4 versions that are interchangeable and may have been fitted as a replacement. There was a later "pan-fill" case and the similar looking C3 that would each have different bells.
However, the starter mounting surface is flush with the block face in each of those. The depth of the starter nose would only have varied with the type of ring gear. The ring gear was welded to the torque converter on most versions.
If you don't have the block plate between the bell and the block, your starter may not be correctly positioned. The cutout on that plate engages the starter nose and sets the distance between gears. If you only have bolts thee could be a lot of variation in that engagement distance. Also, there are shims available to move the starter nose away from the ring gear. You can find them for small block V8's.