Aftermarket. I've pulled my power steering system from the car - typical mid-60's "finger" steering.
Aftermarket systems are nicer, because they incorporate the power-assist function, which adds road feel back to the driver at speed. There are people who put together kits using the Mustang II steering system, which is another alternaitve.
Road feel is lost due to the volume of fluid moved by the pump. You could drop a bunch of money on a KRC pump or you could take the route I did and install a late model Ford pump. It seems to give me great feel for the road, and for $45, it's a good buy. You'll also need to have a high pressure hose custom fabbed ($40).