My MGB came with seat belt guides already fitted. These were the guides made from plastic available from the MG Club. They are quite strong but when I reached over the seat I bent and broke one of the guides.
I bought a similar pair of guides but made from aluminium a lot stronger but smaller. Size may not be a problem but due to their shape sat low directly on top of the top of the seat – which rubbed on the leather which i was concerned would eventually damage it.
So fortunately I hadn’t thrown away the broken plastic guides so I modified what was left of them to convert them into a spacer to hold the belt away from the top of the seat backs.
I think the pictures explain the mods.