Rear Bumper brackets- 65' Chevy C10

[booga]

Stupid question, but I gotta ask. Do the Rear bumper Brackets mount on the inside or the outside of the frame? The reason I ask is that the previous owner welded a cross member to the frame to hold up the gas tank and did not reattach rear bumper if he had one. Thus eliminating the possibility of using "factory" brackets if it is mounted on the inside of the frame.

Thanks in advance for your input.