It would be a sqeeze. I'm pretty sure that the I6 is longer than a small block. And probably taller. Considering that the 4.3 is very similar to a small block, why not use one? I6 is a good idea just not as cost effective. Especially since you would almost have to buy a new engine. I can't see very many totaled trailblazers at the junkyard with good engines in them.
You can get a LT1 out of a junked camaro with the tranny for less than you would get the new I6 engine for.