Hunter-Reay had to hold off Scott Dixon at the end, and Dixon settled for second for the fourth consecutive year at Barber.
"I'm going to go buy a bridesmaid's dress tonight and party pretty hard," joked Dixon, who had to charge through the field at St. Pete to finish fifth. "It's still good points. It's good at the minute, but you know, it would really be nice to win at this place sometime."
Castroneves was third and thrilled to take over the IndyCar points lead.
"You have to enjoy the moment, but work for it," he said. "Those points are very valuable if you are thinking about the long-term. Last year the championship was lost by, what, (four) points? So you got to take as it comes. We've got to keep putting ourselves in those opportunities and the win is going to come. With that, I'm going to add more points."
Charlie Kimball, Dixon's teammate at Chip Ganassi Racing, was a career-best fourth as Honda split the top four with Chevrolet one race after Chevy swept the podium.
"I think Chip said it best when he said it's my third season, it's time to start winning," said Kimball, who with three laps led Sunday is the only Ganassi driver to lead laps this season.
"Our expectation is to be consistently in the top five, qualify consistently in the top 10, and come away from the season with a podium and a race win or two."
Power, the two-time defending race winner, wound up a surprising fifth after failing to contend all weekend. His Penske team tried initially to complete the race on an alternate two-stop strategy, but never got the yellow flag needed to make it work.