I got one of the Offenhauser intakes, and I assumed (wrongly) that I could use my stock air cleaners on it. They are about 11.5" diameter and the carbs are 10 inches on center. Short of cutting and welding I'm wondering if there are any nice-looking (i.e. stock looking) alternatives?

Use 2 of the 62-65 factory 1bbl air cleaner assemblies.
DD

Weird, I must have the wrong air cleaner because mine is bigger than those. It's 11 inches in diameter. And my car is a 64 Valiant that has been in the family since new. Someone must have swapped it???

Yes, It sounds like you have the later unit.
The thing to watch-out for is the top "bail" that mounts to the carb & holds the screw stud for the air cleaner assembly. The "small & tall" air cleaner uses a taller "bail" to hold it to the carb.
DD

Thanks. More parts to add to my shopping list.