Correct on the clearance part. It is in some retainers more common to 360 heads. If your retainers are solid then your good to go. The shim was on my heads when I changed my springs and they were stock. I'm not totally sure why those are there other than a wear point for the spring against the head in case there is some rotation. Maybe someone else can chime in here and explain those shims. You might consider a spreadbore intake with a non smogger thermoquad carb. I port matched my stock 318 heads to match the intake off a 340 and rebuilt the carb. Mine runs great!