it really depends on the thickness of the flanges and how rusty the stock nuts/bolts/studs are. if the flanges are thicker than stock, the bolts may not be long enough. i honestly don't know the thread size or anything, but why not remove one stock nut and/or bolt and take it to the hardware store to find out?
it wouldn't hurt to use fresh hardware, especially because the high temperature application causes the corrosion coating on the hardware to breakdown quickly, allowing them to rust eventually.