I do not know about Monte Carlo, but other GM..if there is a stamp on the frame rail, which I would think there is, it would be on the top of the frame rail and on the drivers side ,under the body, and/or between the body and frame as not to get tampered with. You will need a small mirror. Also I would think it to be under the drivers seat on the frame rail.. Also it will be stamped close to the edge of the up and down side part of the frame, about 1/4 of inch or closer to the up and down side part of the frame, which I guess a C channel frame maybe on top closer to the outside of side of car. It would be stamped close to the side part as this would be the stiffed or strongest part to resist the pounding of the punch in order to indent the numbers, it will most likey not be in the middle of the top of the frame from side to side as it would be to springing to the punch. feeling with fingers on top of frame and under body from front to back of car, close to side you can feel a small dent about 1 1/2 inch long which with mirror may show numbers in a row. take you time and you can feel the indent right at the edge. also you will need a mirror to see it. numbers are about 1/4 high.
I have done this and found it, the 1 1/2 inch long and 1/4 inch high,, dent is the give away on top of the frame, when you are feeling for it..
I have also heard of numbers being in body under trim on bottom of rear deck glass, bad thing is could be coverd by sealer to glass. also on rear frame rail, also on top of rear axle frame hump. Most likely access to number when wheel is off???? again I would guess drivers side on top and on the close to the edge and on the top.
I would not want to be you right now good luck
if you find we will know because we all will hear the YAW HOO
stano