My opinion?
Pay Attention to what Doug Rainer wrote
Or suffer nightmares of ground loop magic in the future.
90-grit sandpaper.
New self-tapping screw for frame
NEW NEW NEW NEW aluminum lug for the big wire. The old lug has no tin left and it is a guaranteed land mine. Repeat GUARANTEED.
Strip wire insulation so that BRAND NEW PENNY colored copper fits into the lug.
SHINE the frame rail under the new lug to bright steel condition.
Tighten the snot out of the lug screw then the frame screw Go to Wal-Mart and buy a spray can of black Rust-O-Leum BED COATING. The coating will last ten years. No paint can match it for durability.
Doing this repair right is as easy as doing it half-assed. And doing it half-assed will cause problems. Absolutely guaranteed.
On Quicksilver I soldered CAST COPPER LUGS onto the No 1 battery cable then I BRAZED the lug to the steel frame. Back then there was no Rust-O-Leum so I used asphalt-based undercoating.
THIRTY YEARS AGO.
Do it once. Do it right.