[fox46]

I would suggest that you use a pair of nice big set screws set at 90 degrees from each other and/or key the shaft and gear, then use a keystock to secure them. But without overanalyzing the question, yes a weld will hold. I would be more worried about stripping the gears with that much force on them. What about using a pair of sprockets with #25 roller chain? With gears, you have 300lbs of shear force focused on a few metal teeth, with chain, this force is distributed along the circumference of the sprocket. But then again, I'm not an engineer.