When I replaced my valley pan gasket, I spent a lot of time forming it, so that when the locater holes were lined up, the gasket rested there comfortably, during the "dry" fit.

Then, I took it back out, and ran a bead of sealant on the heads and block, where the gasket contacts them, put the gasket on, and pressed it into place. Then, I sealed the topside, around the edges, and ports.

I didn't use the rubber front and rear seals that came with the gasket kit. Instead, I ran a thick (about 3/8" high) bead of ultra black silicone sealant. You can run several layers, or use a back and forth "cake icing" technique.

Then, I placed the intake manifold in place, put the bolts in finger tight, and let it sit for about a half hour. After that, I torqued them up.

That was a year ago. So far, so good.

Since then, I've discovered "The Right Stuff", by Permatex. You may want to look into using that.