My first thought is the same as Scott's. But just for laugh's, what V-6 are you thinking about?
The 4.3 is really just a 350 with two fewer cylinders and its just as wide as the 350 where it matters.
But a 2.8 is a bit narrower and might fit without interference? I'm not sure why anyone would want a 2.8 in a truck, but perhaps it would fit?