I use an Ilex #4 with my 19" red dot. Make sure to purchase a shutter tester and calibrate the shutter. You should calibrate every shutter you use especially when shooting 12x20 with the cost of film. I put pencil marks on the shutter dial for the speeds e.g. 1, 1/2, 1/4, 1/8, etc. All Ilex shutters vary in actual speed from the markings on the dial regardless of when they have last been CLAed. A good service technician will give you a calibration sheet when a shutter is CLAed, but Ilex shutters may vary with use or age. You should always fire an Ilex shutter a couple of times at the desired speed before exposure to get the most accurate exposure. Copal shutters are more accurate but are much more expensive. Pay someone to do the mounting. Other posts here mention a couple of options.