There are plenty of websites and Magazines in the UK that discuss traction engines of various types here's the only ones I found in the USA.http://www.steamtraction.com/http://www.smokstak.com/forum/forumdisplay.php?s=cfb3baaeee65052c4f2c3a96c3791dc6&f=7
You also may find that your boiler will need to be pressure tested by some officials, that's the way it is here in the UK.
I expect balance will be an issue with putting a steam engine onto a spider so you need to think about that.
One thing that might help is a book I got a many years ago from Boston Computer Museum called "A walking robot" which had a hydraulic recovery system which although increased the complexity of the hydraulics reduced the power needed to make it walk. The idea is that as one leg moves down the energy is recovered from that to move another leg up.