The JD423 dozer blade requires SVC hydraulics. IF there are enough ports on the SCV block to add the belly mower height controls, then the only question is the longer dozer blade mounting pivot arm that runs to the mid-section of the JD850. My Yanmar YM2610 is a JD850, yet it has the PowerShift (automatic like).
What belly mower are you using? JD72 or JD160 typically are used in the 50, 55 and 70 series tractor mid-mowers.