I'm with twosmallwheels on this one. The Monte Carlo has a shorter wheel base and all older Monte's have more motor set back than do the Impalas. That'll shorten the shaft even more.
Its best to have one made up. In a pinch you can use a shorter one (no more than 1/8 in.).
When I was new to this game I put a used driveshaft in my 68 Impala and it was a huge headache finding one that fit with the right size ends for the two U-joints. (I actually snapped the end off my driveshaft so I HAD to replace it) If you need to replace it, back then (about 15 years ago) Napa sold new drive shafts balanced, with the correct u-joints for $185. Look for a new one. I spent almost that much money and at least 10 times the labor going the used route. The driveshafts from various vehicles are very similar in length, but it has to be exact, as well as be the right size for your transmission (TH400, TH350, etc) If your drive shaft ain't broken, I agree with the previous post. Have it re-done.