Starter Problem

May be a silly question, but what kind of transmission did this engine have behind it, before you got it?

It is possible that the indexing dowels in the block are missing? A manual transmission indexing dowel usually doesn't cause starter problems, because it's only a few thousandths of an inch, but missing them entirely could easily cause an improper tooth engage, from the bellhousing not aligning.

You mentioned that the prior owner mentioned the engine started hard.

I had a transmission come back from someone who kept having trouble destroying pump drive bearings because the engine had offset indexing dowels in it. Indexing dowels are often overlooked. Sometimes they are completely missing or damaged if a block has been set on them, upright in storage, etc.

I would also compare your starter to another starter, in terms of indexing size at the tip of the starter gear housing.

The starter does not align with it's bolts. Chrysler starters align with the end of the starter gear housing, into the index on the bellhousing, directly behind it. You might want to check and make sure that the bellhousing is free of any contaminants in that indexing hole and also check to make sure the starter has the right size diameter and depth of circle on the end of the gear housing.

I would suspect the starter first, if you are running an aftermarket starter. Compare it to an OE casing, check the diameter of the end of the gear housing that has the round index with a dial caliper and check the transmission casing for contaminants or signs of it not completely indexing. If the starter has a small index circle on it's end, sometimes it can start nicely and eventually move away from the ring gear, because of the same reason you suspected the bolt being the culprite.

When compared to an OE starter/ casing, sometimes they require a collar to get that gear engaged with the teeth if it's shallow in tooth depth. Or, if the gear is not engaging in thrust, could require drilling the trans case or removing some material from the end of the starter gear housing. It looks like the starter is mounting flush from the side that I can see in your pics, but I couldn't tell you without seeing or feeling it against the transmission, if it is.

My money is on the starter indexing end not fitting the trans properly.