I feel your pain. By far and bar none, finding the correct air cleaner, heat stove pipe, and exhaust manifold with heat shield was the most difficult part of putting together the correct 327 / 210 hp engine for my '69 RS.
You are correct, in that these items do not exist in the aftermarket - not highly demanded. Suggest eBay and the various swap meets. I found the correct mainfold, with heat shield attached, at Hot August Nights in Reno back in 2003. Air cleaner and heat stove pipe I found at salvage yards. I also found (and still have) a '68 air cleaner for the base 327 / 210 hp set up. I assume that you have the air cleaner?