just run auto gaskets or just put a good size bead of high temp silicon. Make sure both flange surfaces are clean, smooth and flat as possible. Put about a 1/4" diameter bead on the center of the flange, lay it down flat some with a wet finger so the silicone does'nt stick to your finger to much, let it sit for about 20min. +/- so it it just skins over a little and slightly stiffens so when you install the manifold it does'nt ooze out excessivley. Carefully place/install the manifold and draw it down evenly but not all the way down, just so the silicone is clearly making contact all the way around the perimeter of the flange area, you will see it start to push outward as you draw the nuts down. Let it sit like this in the sun (heat) to cure the silicon more quickly. Let sit for a couple hours. Then snug them down untill they seat firmly againt the head but no need to over tighten. Over tightening can draw the flanges down at the nuts and warp them  some making sealing more troublesome. Then complete the connection of the exhaust hose and water lines. Manifolds should not be moving during exh. hose install. You should be able to fire the motor for a minute at this point and put some heat into the exhaust flange area but not to much and just idle the motor. Exhaust gas pressure increases with motor rpm. You should be able to head to the water at this point. Keep a wrench handy (9/16"?) for the nuts and check for snugness just before launching. Maybe check again later in the day after running for a while to insure they don't loosen some from expansion/contraction from heat cycles. If done correctly it should last you untill the next time you have to remove them for some reason. Having a tube of silicone (std. and hi temp.) handy is always a good idea with a boat. Goos luck, hope this helps.