Just order this part number from any local GM parts counter.

24502540 Adapter - V8 Rocker Cover
This adapter is used to bolt flange mount (1958-87) valve cover to 1987 and up center bolt style engines.

Technical Notes: This adapter is CNC machined from billet aluminum stock and this package includes two adapters and fasteners. The replacement o-ring gasket is P/N 12480023.

Adapter - V8 Rocker Cover

I get cost plus 10% from my local dealer and I'm not a business. Talk to them.

Tom