help ID my inline 6

[larsguitars]

Well, I've been trying to install headers for 250 CI engine on my 65 c10. They just don't fit.
At this point I have cut the motor mount support out and The headers still won't fit. So I'm wondering just what size engine I have? I was told by previous owner that it is a 250 CI. So I have looked online to see if I can figure out what it is but, can't find my engine numbers on any of the lists. Help!
The engine looks like a 250 or 230? The motor mounts are in the center of the block and the fuel pump is at the right front. But I'm just not sure.

Block casting number is, 8877178G
The number behind distributor is, F0126FM If anyone knows how to ID my engine I'll order the correct headers.
THANKS!