I thought you meant 550 Front Wheel Horsepower when you wrote 550fwhp
Any ways, I would also worry about the tranny/rear end too. I think they say a stock block handles up to 400 hp easily but then its a toss of the coin after that.
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1993 Mustang GT - March Ram Air & Pullies, K&N, C&L 73mm MAF, Super Competition Hooker Headers, 2 1/2 in. mendrel bent cat back, High Flow H-Pipe, Pro 5.0 Shifter