GMC items such as what you're looking for are pretty scarce in today's times because there weren't that many made for these engines back in the day. Patrick's Antiques in Casa Grande, Arizona also has a few GMC items they offer, so you could also add them to your search list. They could also have some used parts as well.
Don Ferguson Jr. #339 used to cast aluminum GMC side and rocker arm covers with Wayne insignia on them. He has the original Wayne 12 port head patterns but they fit the standard GMC heads as well. I purchased a set in 2,000 from him. He may still have some available. Don is at 310-379-4720 according to the 2015 membership roster. He also has the patterns for the Arias 12 port head and Arden heads for the Ford V8.The Ferguson family has been racing GMCs for generations and hold many land speed records.
Don Ferguson Jr. #339 used to cast aluminum GMC side and rocker arm covers with Wayne insignia on them. He has the original Wayne 12 port head patterns but they fit the standard GMC heads as well. I purchased a set in 2,000 from him. He may still have some available. Don is at 310-379-4720 according to the 2015 membership roster. He also has the patterns for the Arias 12 port head and Arden heads for the Ford V8.The Ferguson family has been racing GMCs for generations and hold many land speed records.
The two things on the valve cover are after market breathers. The long curved tube going into the block is the stock oil filler tube that also has a breather/ filter in the canister.on top.
Because this market is such a tiny segment of the aftermarket, parts like you are looking for are going to be be somewhat higher than you'd expect simply because no one is producing this stuff in any kind of quantity. There is very little demand, thus very little supply but higher costs.