A518 verse 727 transmission question

Hey guys
I currently have an A518 transmission behind my turbo small block in my 1974 aussie valiant charger. I'm making just over 600hp at the wheels. I'm not sure how much horsepower the A518 can handle and I'm not sure how long it will live at that horse power. I would love to keep the Overdrive for cruising on the highway. I have noticed that a lot of the bigger horsepower cars have the 727 not the A518.
I know they similar to a 727 transmission with an Overdrive on the back but how similar are they. Do they use the same parts to build them and can it handle the same horsepower. Can anyone point me in the right direction to work out what needs to be done to my transmission to handle that horsepower and if it's even worth doing.