TH400 behind a 350 OEM, when and why?

[leftybass209]

I've been doing some research about the automatics in these trucks. From what I understand, M49 designates BOTH the Th400 and Th350, and there is nothing specific on the SPID that identifies which transmission was installed.

Short of finding a build sheet, were there any options that could be ordered that support putting a Th400 behind a 350. For instance, a camper package, or optional rear gears?

I ask because i'm going to look at a truck tomorrow that says it has a Th400 behind a 350, and is claimed to be stock; totally possible I understand. I'm interested in WHY this would have been ordered that way, and in what circumstances, if any, it may have been suggested by Chevrolet as a better fit.