I agree with Quick, the floating being the better. It would also less likely to score the rifling
..but,
With a straight walled cartridge like the 44mag being reamed to the longer 445SM, I honestly dont think you'll need a pilot. The reamer will be nearly all the way in the barrel to start, with only having to lengthen the chamber.
It wouldn't be like going from a .22 hornet to a .225 winchester where the bore/rifling would be your only means to guide the reamer because of completely reshaping the chamber itself. ....not just lengthening it.