You should be fine.
With a bulk-head fitting there is enough of the fitting down below to allow you to run the jam nut up by hand and then hold the fitting while you tighten the nut.
When you get a little tension on the fitting, there should be enough friction from the rubber gasket to hold it from turning while you snug it up the rest of the way. If you're using any fittings inside the overflow, a screen is a good idea, put them on before you put the fitting in the tank.
One word of caution, the jam nut doesn't need to be as tight as you can get it, only good and snug. Check it after you fill the tank and again it a couple of days just to be sure. If it's dripping, there's no trouble snugging it up with water in the tank.