OEM-style battery cables for slant 6?

Are you talking about a split, do you mean a terminal like this?
upload_2021-7-13_12-48-25.png

At least on the late 60s cars, this is the connection to the starter relay, and I think the otehr end was a ring terminal. Wire itself should be brown (this reproduction is black - incorrect)

probably the original from my '67
upload_2021-7-13_12-50-52.png

That wire was bundled with the starter cable from the battery.

This is a little more correct in that respect.
upload_2021-7-13_12-53-1.png

FWIW 1969 6 cyl cable routing
upload_2021-7-13_13-23-58.png