Yup - that's a classic tail light.
What year frame are you building the bike on? That's a 1977/78 CB750K tank, and the frame is different on that model than earlier ones. If you want to put on one of the mid-70's classic tanks with the chrome gas cap, it won't fit. Just a heads-up. But, the CB750F tank should fit and looks good. All depends what kind of bike you're building.