Where are the 292 experts?

['81 GMC 3+3]

The simplest things have been not so simple when looking for items for my '81 2500 with a 292 straight 6. Take the air cleaner for example. A short sighted or unresourceful previous owner scrapped the factory air cleaner assembly when they put a new Rochester Monojet in. Now sits a 3+ inch air filter on top, fully exposed, always filthy and in need of replacement every few months.

So, which advice would you give me- Go to a K&N style and find a fit for the top neck of the single barrel, keep slogging through wrecking yards and the internet for an original OR some third option which I will be happy to entertain?

Thanks in advance fellas.