remove my crossmember without worrying about the carraige(or whatever it is) pulling outwards?

 

I don't usually recommend jacking up a classic mustang by the rails, but if you use a 2X6 piece of lumber between the jack and the frame rails, you will be OK. With it in the air, you are not likely to have anything move while the crossmember is being removed. If you are removing it to get the oil pan off, you are going to have to jack it up anyway.

 

The car is currently on jackstands so I wont need to jack it up. The engine is in it so wouldnt the mounts and engine hold it in the correct spot? I forgot about the engine holding it..

Thanks

 

Yeah, you are OK. I had mine off several months ago when I replaced my oil pan and nothing moved on me.