Pitman arm pullers are massive things, and are purpose-built for the job. Find a shop or a parts store that's got one, and rent it. Most of them have some sort of collar or other device to keep the puller jaws from spreading out while you're turning the jackscrew. I use a 3/4" drive impact wrench on mine, and it has trouble removing some of them.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!