Crawl under that Monte and look at the frame. I'd bet its a perimeter type frame and that the outer edges of the frame between the front and rear wheels will run right under the running boards on your '51.
Also, check the wheelbase on the Monte, the later ones are 108 inches, while the pickup is 116. I don't know how long the '70 is but I doubt its exactly right.
As it happens I'm watching OverHaul'n on TV right now and they are doing Lance Armstrong's '70 GTO. I saw the frame, which is probably the same frame as your Monte Carlo, and the one under the Goat is a perimeter type.
Now, if I haven't discouraged you too much, check this link
Little Red Truck Frame Plan to see how crazy I am... I have a '78 Monte frame outside!
