Geoff.
Don't you wish you still had access to some of the items that were scrapped back then? Except for some heavy truck models, most of the air cleaners were only available as an assembly from Ford. The duct and valve (snorkel) were another separate piece on many of the air cleaners since the main housing would be the same for several models but the duct and valve were vehicle specific. That only got worse as the years rolled by. Most of the duct and valves at least had an ink stamp of the engineering number. Occasionally there would be an illegible or missing number. Sure wished Ford had done the same for the lower shell or base. Except for some models such as the 68 GT500 KR, 71-73 Mustangs, and 72 Montego (which used plenum chambers), the majority of Ford ram air cars had the Flapper door in the air cleaner lid. Those usually have three studs on the base to help retain the heavier upper lid/flapper door assembly. Guess they figured the paper tag would suffice as no one figured we would be "Fretting" over these cars nearly fifty years later!
I believe Ford only built a little over 1,500 ram air 71 429 Mustangs, so anyone willing to modify an OE air cleaner is not going to be on a very long list. Not sure what would be necessary to work the air horn issue between the two different carburetors. Believe it or not, most of my friends in our 429 Club that did not have "Shaker" cars pitched their ram-air cleaners. Most felt the air cleaners were a PIA to remove/reinstall when they were racing and wanted to do carb work or fine-tune the engine.
They preferred a simple open-element air cleaner as Ford used on the high performance 271/289. The two "Throw Aways" that makes me cringe the most is one off a 69 Boss 429 and a 70 SCJ Cyclone. Thrown away like they came off a Pinto!!
At this point, unless an OE air cleaner falls out of the sky into your lap. the site canted 393 listed looks to be an only choice. They do offer a 429 base/shell in fiberglass for around $400.00. If your friend is more concerned about functionally over 100% Concours appearance then this may be an option for him.