I second the "go for it". An old Raleigh with 32 spoke rims held up under my 420-ish girth for about 1000 miles, then from there to 380 a Specialized hardrock got 700 miles of abuse. The 520 will likely break AFTER you do
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I also second the job well done. It's a hell of a thing to work off 50 pounds, keep at it!