There's under ten cubic feet of trunk space in a mustang with a soft top. So it's possible to get both big bags in the trunk but it may be tight, where an inch or two in width or depth may make all the difference. I don't think there's any chance of all four bags in the trunk.

There should be no problem putting a couple of the bags in the back seat. I don't believe the back seats have weight sensors in them, if they do just buckle the seat belt first to keep the alarm from going off.