Rare 1967 Mr. Norm's 383 4 speed dart

I think the existing manifolds worked, but with some modifications- with a hammer, lol. Plus wouldn't any modifications to the body to fit existing manifolds cause issues on the assembly line, and possibly with the customer later on?
I'm not sure, but maybe the existing manifolds only worked with automatics, so that could be another reason.

For what its worth, they also shaved the stock driver's side motor mount rear flange off the block, on some cars anyway, to clear the steering (the stock mount used in other body styles).

According to most sources, for the 383 Barracudas 67-69 they made more standards than automatics in all years. When I go to shows I'm always on the look out for 383 cars, especially 4 speeds, and the majority of the time they are autos. If they are 4 speeds, they have a console.

I would guess that more moving parts to break always plays a role in a car's survival.