Raylar makes intakes but mostly for boats and Dart makes one for a four barrel. The stock intake is not bad for low end but quickly runs out of air and needs to modified to get the most out of it.
Read this post on intake info for the 8.1L.
http://www.chevyavalanchefanclub.com/cafcna/index.php?topic=146644.0
http://www.raylarengineering.com/intake_manifold.html