That valley pan is a notorious spot for leaks. On another note, the last two FelPro gasket sets I have used, the valley pan gaskets must have been the ones for the short deck 266/304s. Trying to stretch it to fit a 345/392 block won't work. Its waaaay too short across the lower part of the valley. I know the mess silicone can be, so I will temper this advise. Be sparing with it. If you cannot source the correct gasket for a 345, straight silicone can be used. Its been fine for the last two builds, but here's what I suggest: Lay the pan over the block before you try to seal it and look for the high and low spots. Dolly the pan flat where needed. Once your'e happy with the fit, get a fresh tube of AUTOMOTIVE silicone. Lots to choose from, but leave the white stuff for the bathroom tile in the drawer. I like to use one of those squeezer thingies for the tube. Helps the bead consistent. Go around the outer edge of the pan with a 3/16-1/4" bead. Seems big I know, but when you get all the way around, set it down carefully over the holes. Drop bolts in the corners and give them a couple turns. Push gently on the pan and force it back and forth, side to side. Drop the rest of the bolts in and hand tighten them until they just contact the metal and are applying slight pressure. By now the excess silicone is pushing out. Grab all the rags or paper towels necessary to clean it up. I like to force the excess back into the gap as much as possible. But you'll be wiping about half onto the rags. Ideally, the inside silicone has not squished all over the inside of the block. It hopefully migrated right up to the edge. And hopefully you don't see any airgaps. If you do, smear some more in with a wet finger. At this point, walk away until tomorrow. You could re-check the bolts at that point. Maybe put a half turn on them if needed. But my experience has been it dries up pretty solid. Hope that helps.
