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I didn't for a Weiand Stealth manifold, in fact, the instructions specifically called out NO valley pan, too bad I already bought the valley pan before I bought the manifold...
It depends on if you have exhaust gases going through you intake. The valley pan was put on there to keep the oil from splashing on that hot manifold and crudding up the oil. I doubt if you would even have to worry about it as long as you change your oil regularly.
I cut the old ones down to remove the port seal parts, drill a couple 1/4" holes in the bottom, then install them with regular Fel-Pro Print-to-Seal gaskets.
This has been discussed to death in the past. Do a search and I know of 2 different posts to this question. I agree with wb6vvv, on a stock application you should use it. Ford spent millions of dollars designing these engines and they thought they needed to be there. If Ford would of decided they weren't needed they could of saved millions of dollars. So Ford thought they needed to be there maybe they know something about their 460 that they designed.