A factory option that was about as rare as hen's teeth (on a truck).

There was no factory option "split manifold" or dual exhaust option for a truck 216/235/261. My bet is that the only GM 235 dual-exhaust manifold was available on 53-55 Corvette 235 engines.

Maybe someone knows the Corvette manifold's casting code.

However, it was not uncommon for shops/services to split a manifold and graft-on another down-pipe and make a dual-exhaust manifold.