The thought of putting a kilowatt of heat into a softbox would scare the...shot out of me.
Paul C Buff ships the Einstein lights with both 250 and 150 watt modeling lamps, and strongly cautions against using the 250 watt lamps with any sort of modifier that restricts airflow -- and explicitly includes softboxes in that caution. In the case of the Einstein, the concern is over the unit overheating and shutting itself down before it can damage itself, not over fire.
But a kilowatt? I've seen hairdryers with less heating power -- and you want to put that into a softbox? Hell, I've seen toaster ovens that don't draw that much....
And, safety aside, there's another factor at play. Even a kilowatt bulb isn't anywhere near as bright as a flash, and a softbox can suck up a good amount of light. Even if you did figure out a way to put that torch into a softbox without burning down the house, I don't think you're going to be very happy with the results.
My advice? Splurge for the $280 for a B800. (Actually, I'd advise saving up a bit longer to get the $500 Einstein, but the B800 is still a superlative light.) If nothing else, think of it as a dirt-cheap fire rider on your homeowner's policy.