Yes, the Suzuki Z400, Kawasaki KFX400, and Artic Cat DVX 400 are all identical machines manufactured by Suzuki(minus the Artic Cat's plastics/fenders). So you can use parts from ANY of them. They are cheap and easy to find on ebay or CraigsList.
The 400 spindles will bolt to the 500 A-arms.
The Yamaha YFZ 450 calipers will bolt to the 400 spindles. That is a common brake upgrade for the 400's.
As far as I know the 03-08 YFZ calipers work, and possibly later years but I can't say for sure.
The Honda 450R and YFZ450 front master cylinders also have the same brake line thread pitch as the 400's do.
The Suzuki LTR450 may/may not work, as I am unfamiliar with it.
Those stock MC's are all over ebay, and rebuild kits are cheap. There are also new stock copies for $30 to $50 on average.
It is anyone's guess as to which MC is best, but both are an improvement to the 400's already decent brakes.
I don't have a 500 for measurements of brake lines or thread pitch.
Plus 2", or 3" ss brake lines cost almost if not the same as stock length.
I would go longer just to be safe, as the 500 is wider and longer than the 400.
I have owned a 400 and ridden several, but have not put 400 spindles on a 500.
The 400 rotors are also drilled, which provides better stopping than a solid rotor.
I would also use carbon fiber pads in the 450 calipers, cf pads themselves are a noticeable braking improvement.
All will be a dramatic difference compared to the 500's ancient brakes.
I have read that the 400 spindles slightly widen the 500, so again I would go with a longer line.
You can however mount the 400 spindles and use string or fine wire to mock up the brake lines.
Several companies like Galfer and Spiegler will make custom length lines for you, which is not uncommon.