Crown is the only one I found that made a preformed notch. They are not hard to build though yourself if you are a welder.
Also, I'm not sure what height front tire you are running but Id be concerned a 5" drop will be running the LCA into the ground on small bumps. I was running a 2.5" spindle and a 3" spring with a 27" tall tire and it was pretty much undrivable without tearing up lower control arms.
Here is the thread talking about it.
http://67-72chevytrucks.com/vboard/s...d.php?t=713024