http://www.summitracing.com/search/B.../?autoview=SKU
I'd go 8" x 11" or larger.
Check that your PS cooler isn't going to be in the way of where you want to mount the oil cooler. If it is you can replace the pipe loop with a more efficient rail cooler and mount that wherever you like.... it doesn't nee tons of air so even the small opening beside the nose bridge of the 89+ bumper will do.