Seems like a strange place for oil to accumulate -- between the 3/4 intake runners.
Any ideas on where it's coming from?

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Did you spill your beer again?

Read my recent thread on oil leaks.

This looks like the similar spot I am experiencing on both sides. Since I just did a new rebuild my tins are spotless but nevertheless under them in that spot are little puddles of oil.
Conventional wisdom says its the valve cover gaskets.

On pelican's forums there is an article of how to address this by welding 24 gauge. steel onto the inner lip of the valve covers to keep the cork gaskets in place.

I should be getting my materials shortly and will be doing this. I will report back when I am finished and hopefully have nothing but stellar results to share.

Thanks, that's probably it.