Those are shoulder bolts with a certain diameter shoulder and length, make sure you get an exact replacement from the dealer or junkyard. For God's sake don't buy any old bolt from the local bolt shop, these are specifically designed and manufactured bolts made just for this application.
Sorry I don't have the size of thread for you but I do have a 400M in the garage with the valve covers off right now and if these engines are the same I could pull one for you and give you the size? Does anyone know if they are the same?
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo