i've read your last two posts and have some questions:
Why do you need H-bridge?
How often you have to reverse?
How fast you have to reverse?
You could use single transistor to control speed of the motor - from
your basic stamp directly or by PWM circuit like this:
http://www.solorb.com/elect/solarcirc/pwm2/pwm3.jpg
by changing voltage of the pin 6 of the LM324 you control the duty
cycle of the PWM. This can be manual or automated...
MOSFETs are cheap, instead of H bridge just use switch to swap motor wires for running in reverse.
if you want steering mechanism then why not use servo?
i know, the cart is some 500lbs...but...an 18-wheeler truck weighs
way more but truck driver doesn't have to be able to lift it - just strong enough to steer.