Head bolts are 5/16 X18. You can buy grade 8 from Fastenal. sometimes from Home depot or Lowes. Sears will also have them. If I remember correctly, the bolts are an odd length probably because Chrysler got a deal on 100,000 of 1/8 inch shorter. Just go up to the next 1/4 inch length.
I can not make out in the photos if the engine is a 105 or a 120. If it is a 120, double check to be certain it is NOT a Magna Power II.
Also, that lower unit is getting difficult to find props to fit. They are the thrust pin 18 spline type. None available from manufacturers. Some on ebay but most are way overpriced. The two piece lower is (as I have proved in a post a couple of years ago) About 3 MPH slower than the one piece. If in the future you are interested in converting (assuming the engine block is running well. you can swap to a one piece lower if you change the midleg. This can be done with the engine still mounted on the transom. Not only do you gain some speed but you have a better selection of props and they are easier to find.