I think the record in the Steinborn lift is Mid 400's, or even #500 or so.
if you've got a good pair of heels, you could try the old fashioned leg press

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That's Wilber Miller a World class deadlifter from the 60's doing #505 for reps.
And Here's Ed Zercher, of Zercher Squat fame-

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Things were tougher back then:).