How do I mount an oil pressure sending unit on an LS swap using the oil cooler block-off plate location?

Use a block-off plate with a tapped port. The oil cooler block-off plate bolts to the pan just above the filter, and versions exist with holes ready for a sender or gauge line:

• GM (Chevrolet Performance) oil cooler delete plate with a tapped hole, or a plain one without.
• Summit Racing billet plates with dual 1/8 in ports, dual 1/8 in 90° ports, or dual 16 mm ports. The extra port can also feed a turbo or centrifugal supercharger.

Make sure a plate is actually installed — they're often missing from used pans, and the engine won't hold pressure without one.