Craig,
To give you an idea of what equipment you would need to convert your 861 from manual to power steering, have a look at ebay item number 280762223586.
Ebay also offers power steering kits for later model Ford tractors (2000, 3000's etc.), but unfortunately not for the 800 series Fords. These typically cost around $600. I have no idea how well they function. It appears that these are power assist steering kits.
Another place to look at is
Jackson Power Steering*|*Customized Power Steering For Tractors And Equipment