I was going to suggest a Maclaren as well. Mine is an absolute workhorse, and while my oldest hasn't topped 50 lbs yet, I have no doubt my Mac will be able to handle it.
Ooo - coolness. I did a quick Google on "stroller weight limits" and found out that at strollers.com you can sort by type and weight limits: http://www.strollers.com/
Just use the left side bar to sort "strollers by capacity", select single, and then scroll *way* down on the left to sort by weight - they show 116 strollers in the 50-70 pound range, and 30 in the 70 pounds or more range.