Definitely trace it first....and keep your fingers crossed that it's an intake or valve cover gasket leaking.
If it's a 427, I would imagine they used a Dart or other aftermarket block, which uses a one piece rear main seal.
I just assembled a 445ci Windsor with a Dart block and left the rear main seal out until last because they just slide over the crank and fit in a seal bore. Big mistake. I had to drop the
oil pan and drop the rear main cap so that the seal would go in squarely and without scuffing it.
If it does turn out to be a rear main seal, let me know and I'll send you one. I have an extra in my garage...and they're anywhere from $20-30 if you buy them from an auto parts store.