valley pans aren't reusable in the strictest since. Most have sealing ridges around the ports and water passaages. Once you torque the manifold, you compress the ridges to seal the intake.
I would reuse a fiber intake gasket before a metal pan gasket. You can trim the sides off the pan and use just the pan portion with fiber gaskets. Will leak a little oil into the pan, but does shield the bottom of the intake from the hot oil.
I don't know why you couldn't reuse a valley pan, however on a dry intake cleveland (no water in the intake) I'd trim the valley pan and just use the lower portion as was stated to keep the oil off the intake. Seal around the ports with some silicone, and away you go. That's all I've used on my clevelands, and no vacuum/oil leaks, and if you are NOT liberal with the silicone, it doesn't squirt into the valley or intake port. Just my .02