Butch,
I'm no JD expert but I'm willing to guess that the 420 has hydraulic lift for the mower deck and auxilliary hydraulic circuits also.

If so, then it will be equipped with a hydraulic pump to look after all these things, including the power steering. The hydrostatic pump will be used exclusively to drive the tractor in forward and reverse and has nothing to do with the other hydraulic circuits.

In order to put power steering on your Massey, you will have to add a pump. The easiest way to do this would be to install a belt driven pump from a car.

Connecting it all up isn't very difficult. The hydrostatic control module that you will be fitting your steering wheel to, should have four connections on the bottom of it. One connection goes to the pressure side of the power steering pump and the other one to the return side. The other two lines go to the steering cylinder. If the tractor turns left when you turn the wheel right, just switch the two steering cylinder lines at the control module.

BTW, what did all of that cost you?

The hydro static pump operates the mower lift and 3 point hitch on my ole MF same as it does on the JD's. In fact, I think they both use the same hydro-- Sunstrand HT90. I guess what I need to do is find a parts picture of the JD's hydralic system and I think I can figure out how to do it from there. I made a goof too, it's from a 400 JD, and is actually a power assist type of steering so I'll be able to use the original steering sector and tie rods. I've had my ole 1855MF since 94, bought it from the dealer that had taken it home new for his own use. My only complaint is that you can't hardly turn it with the FEL bucket full of sand-dirt-etc. I bought a 445JD today so can use it while I'm redoing my ole 1855, one way or another I'm going to put power steering on her. I'm planning on completely stripping the tractor-new paint, new motor, and rebuild the hydro. At over 2500 hard hrs it deserves a little TLC and I plan to give it to her.