Transmission-bell housing question

[Lucchetti]

Recently I broke the Bell housing/ flywheel cover. I would like to replace the part but I don't know the size of my transmission. How do I figure this out OR how do I replace the old part without knowing the transmission size?

History-I just bought a 1968 c10 short bed. When the kid sold it to me he said it had a 283 engine, and that's about all I know. (I’m not even sure that is correct.) As you can tell I'm a little green at this but I do plan to learn my way around and get this truck rebuilt. I appreciate any help on the bell housing questions for now.....thanks