Please be sure to use the proper base gasket if you have the powerhead off also.  The “spring model” 7.5hps up to and including 1955 used one type of base gasket, later models with the rubber shock used a different base gasket.  Mix them up and you will have an overheat problem…

Like Aquasonic says, go ahead and use a gasket on both sides of the exhaust baffle.