DO you have access to a PC with Solid works and COsmos... a friend of mine made all stress calculations of our graduation robot on this software. if you'r interrested, tell me and i'll put you in touch with him.
hello, i have a detailed tutorial on my website explaining how to build a compact and powerfull H bridge.
http://www.ikalogic.com/H_bridge_1.php
feel free to ask any questions about it.
good luck