How do you tell what liter your Slant 6 is?

I can tell you what not to do. Don't try to get it out with a cheap Harbor Freight "easy out". They invariably break off in the bolt and then you have a piece of hard steel you can't drill. I did that on my 383. My best luck has been to drill a pilot hole directly in the center of the bolt, then gradually bigger, then use the correct tap to get the bolt out in pieces. You might also drill it out enough where you can peel the remainder inward with a little screwdriver. The trick is in getting a centered starting hole. Some machine shops have a "tap burner" that vaporizes even hardened steel electrically, but you must remove the head and bring to the shop.