The 740 has the same width/height as the 700 and 720, so will fit in the same area as your 720. The 740 is a bit thicker to accomodate the new screen and battery, and the rear mount has been changed from the KeyHole style to a 4-prong Richter mount. We have a RAM Adapter for this, and we have a flat plate you can use to secure it to a panel. If you have the 700/720 AirGizmo it fits - but not perfectly, you don't get a solid "snap" - works better if you snap it in the opposite direction of the 720.
Yes, the battery works fine on the 740. Because of the increased power draw it won't last as long, perhaps 2 to 2.5 hours, but this is in addition to the ~30 mins of internal power you get with the 740's internal battery. Of course this can be extended by dimming the screen or turning the LCD off.
Hope this helps,
Walter