I`ve seen many with small cracks like this and I have a few recommendations for you that worked for me and several others.
Make sure you are using the correct bolts with no washers, no shims. Make sure they are spotlessly clean. Check the milled flats when the bolts go on the starter and make sure they are also spotlessly clean. Install the starter, torque the bolts to 40 ft lbs. Next, going by the picture as a guide, by all means, add grounds. 1 ground going to the alternator bracket don`t cut it, especially if the engine has been out of the vehicle and likely the factory grounds were not reconnected elsewhere. Run the ground to the block and another ground from the block to the frame. This may not seem important, but when you factor in how less harder the starter has to work to turn the engine over it`s a big benefit. Here`s what I do to all vehicles I work on with starter issues ground wise. I make sure the ground from the firewall to the engine is still connected, I add a ground from the battery to the fender if it`s absent. Ground to the front of the block then to the frame. I remove the brake line holder, clean it spotless, then add the ground with the brake line holder, bolt it back up and go. In every case I`ve seen a difference, the charging system works alot more efficient, the head lights brighter, and best of all no starter trouble. Since chevy`s use the solenoid on top they get hot, so they need all the assistance they can get, reducing the resistance by adding grounds usually does the trick. My cutlass header was riding directly on the starter, after going through 8 starters and doing what I just said, no more starter trouble since.