How about you give this a shot ?
onready var aimcast= get_parent().get_node("AimCast")
I'm pretty new myself and not sure if it'll be something that's considered a good practice but you can save the AimCast as sperate branch by right clicking on it and selecting "Save Branch as Scene" and then instancing that to your scene where you want that. Then you can make that AimCast Local and make necessary adjustments or changes to it locally there. This way the above code should also work.