Not sure anything out of the combine would work with the exception of the pump, which is a two stage pump. Both the front stage outlet and stator outlet need to be Tee'd together on a combine pump. The 500B/600B used a Char-lynn steering motor mounted in line with the steering shaft. The motor is located under the carb. The steering shafts need to be modified in a machine shop to fit the motor couplings and you will have to have all of the correct yolks, etc.
Unless you want a real cobbled up job using the combine components, not saying it can't be done, I would get a system out of a 300B/400B or 500B/600B tractor as they are the same I believe.
Joe Young recently converted his 300B to power steering and it took a bit of machine shop work on the shafts even though he had the proper pump and steering motor.
John Saeli sells the Eaton/Char-Lynn motors for these tractors and you will have to find the Case pump/reservoir pt. no. G45231, J.C. Marleau has one for a reasonable price. The pump is a contract John S. Barnes unit (Barnes is now Haldex).
Hope this helps.