You have a v6 in a full size car? thats a first for me. Any chev v8 will fit in that engine bay, new mounts/trans/ and rad are the basics. Smallest motor I have ever found in the Impala was a 305, but then again anything is possible. i used to see those in monza`s
that car must be slugish i have an 1985 impala 4dr it has the 305 V8 and it has 165 hp stock it is so slow. It is really up to you the size is pretty much unlimited you could put in a crate 454, 427, 396... you get my point. the only thing is the bigger the engine the more power you are going to have and the stock trans and diff aren,t going to be abe to take it.
Simplest engine to drop in with out major mods is a 350 (5.7L). It will fit, may need different motor mounts but all should bolt up easily. And while you are at it drop a TH350 tranny in as well. You will have the best power and fuel economy you can get from the car.
The 454 will need to have major mods done in order to fit.
And yes there are a few 5th generation Impalas with a V-6 fortunately not many but some.
the only difference between a 305 and a 350 is the bore on the 305 is smaller than the 350. motor mounts, trans, radiator all fit. mine is a 305 with a 200r4 trans. my plan is to drop a 396 or 427 with a 6 spd trans.
I would go with the 350. It is probably the most economical, in both mileage, and engine cost. I had the 305 in my car, and it did pretty well. Not the most powerful thing, but there are ways to get a 305 to move!