Dartin for Divorce

Purple power or oil eater is great for oil. On porous blocks, @Bodyperson is right: solvents. A scrub brush helps too.

The speedo sender should have an O ring and a lip seal on the speedo pinion. Are you leaking from the sender or from where the cable connects?

Most autozones I've been to carry the thicker dual plane (two ovals) gaskets in the performance and chrome aisle, but ymmv, it's a regional thing.