I have not worked with a Mack Bros action. There isn't much info on their website to know if their mag cut is for a 3" box. It looks like they use a trigger hanger which may reduce the available cut at the mag well. Sounds like you have it in hand. You could take measurements from the front of the mag box cut to the trigger hanger to see if there is enough room for the OD of a Bainey box. You would need to check the same measurement to the leading edge of the bolt when the bolt is back against the bolt release.