I just wanted to know if I can install a camshaft out of a 351m/400 into a 351 cleveland? The engine is out of a 1977 Mercury Cougar and the Cleveland is a 1972 2V. Are the firing orders the same between the 2 engines? I know that the engines are from the same 335 family. Im not looking for any performance just want to get the Cleveland going again.

Yes you can use the '72 cam.
For best performance, get a chain and sprocket set with a multi-position feature and install the cam so it is 4 degrees advanced. It will work with a stock set, though.

The firing order is the same 1-3-7-2-6-5-4-8