Rebarrelling a 99 is very complicated, compared with most bolt action rifles. The cut for the positioning finger on the top of the bolt, and the extractor cut must be made. While the extractor can be removed while breeching is being finalized, the finger cut on the top must be made before the bolt can be closed. That is, the barrel indexing must be finished before headspace is set. And the barrel must be removed to use a reamer - unless a custom pull reamer is available. One chance to get it right. In addition, the barrel contour must work with the forend, and provision must be made for attaching the forend. A while back I did the final installation of a .22-.250 barrel on a .250 action. The square threads had already been cut. It was a very time consuming job, but worked out well and the owner was happy. Not one I'd want to undertake again.
It would be much, much easier, and likely cheaper, to sell yours and buy a .250 or .243.