My name is Carl Carter and im fairly new to the z world. I picked up a 77 280z and rebuilt the engine but i found out its a l24 block with a 280z head and i have no idea why somebody would do this but the person who built this engine knew what they were doing

Welcome to the forum, lots of good information in on here. I personally haven't a clue why soemone would due that but I'm sure there was some kind of reasoning behind it (even if it's the finical aspect and not a performance)

What head and block did he use?

Post some pictures of your car and have fun in the "Z world".

It was probably what he found or had laying around. But the 280Z head does breath better, so it might not be a bad combination.