Just saw this thread! Here's a direct link to the page about building a pyrometer: http://www.caffreyknives.net/forge_pyrometer.html I have one in each of my 3 forges. The thermocouple listed on the webpage works well for a general purpose forge.
For a welding forge, I would recommend this thermocouple: http://www.omega.com/pptst/KQXL_NQXL.html The thermocouple I listed on the webpage is rated at a lower temp, and if used constantly at welding temps, it will burn out within a few months. This one last MUCH longer (but its a bit more expensive)
On my home page, there is a drop down list of links, a green box that says "Links to entire Website". Click on that and scroll down, the link to the pyrometer page is near the bottom.