Starter Problem

If the starter ears engage the transmission casing completely flush, as they appear to be, the thrust depth is correct, but you may still have an issue with the alignment index of the starter being too small for the casing or possibly off center, or coming in at a slight angle (explained below). It should be dead center with the gear shaft, on the inside of the gear housing, that holds the gear, all the way through the end. If the gear shaft hole is not centered in the starter case, it could have been machined off. Check for runout/ off center gear shaft on the starter to that end index, as well as the diameter. All it takes to know is to use a dial caliper to measure the diameter of the index on the starter and measure the inner diameter of the trans casing, with a drill bit or caliper, if you can get it in there.

You may want to try another starter in it's place to see if you are experiencing a problem with the starter, itself.

I've also seen starter motor flexing on the aluminum starter case, where the motor of the starter itself was contacting exhaust, contacting engine to trans bracing or contacting the block itself. The ear can be flush and you can still have out of square issues with the motor and gear shaft itself if the motor or any other part of the starter is contacting anything at all, other than the mounting surface of the trans. Be sure it's not.

I also know that some transmissions have sleeves installed in the starter index. Some manual bellhousings actually have moveable indexing cups, entirely for full adjustment.

Check easy stuff first, like motor/ housing clearances against the block, exhaust, braces, etc, then disconnect the starter if that looks ok, to check it's indexing stuff.