The factory shroud off the big trucks may be a good start already has the 18" diameter center for the 216/235 fans. I have sold three this year for around 40-60 a pop on EBay here are a couple pictures and the last description from the one I recently sold.
This is an original fan shroud for Chevrolet trucks. It bolts to to the 6-radiator attachment bolts. These carry part #3705063 and are suitable for all models of truck except the 5000 series COE. These make a significant cooling difference in a stock motored street/show truck of about 10 deg. Best thing is they are factory so period and points correct.
http://good-times.webshots.com/photo/2705488650102454676sLCmCAhttp://good-times.webshots.com/photo/2673348600102454676qtmVHy
