Can't you get a smaller bar to fit the holes?

Otherwise, I would drill a 2" hole in a piece of wood and then clamp that in place as securely as possible over the work piece. The hole in the wood will be the guide for the hole saw.

You might need to check out an equipment rental place to get an adequate hole saw. If you can do the job in a few hours, you may be able to just rent a saw for half a day. My local place will rent to you for a half day if you get it back quick enough.