People seem to like the DSE stuff too.
http://www.detroitspeed.com/productpages/steering.htm
You can roll the dice on an IROC/WS6 box. The fittings change, so it may be advantageous to get a pump & lines too. That is what I am doing in my 77, at least for now. I pulled it from a low mileage WS6 85. The problem is getting them over the counter at the parts place. They are often the wrong ratio and rebuilds are sketchy.
Unknown used steering boxes are not worth any effort. They are too big a PITA if you find out it is bad or not the box you want. Just my $.02