finding/building a junkyard 360?

Go Magnum since you have one.

Things to avoid;

Caked up/burnt oil under the valve covers on the valve springs and cloges return passages. The valve cover is probably a joke and cakes with burnt oil.
Sludge in oil pan.
Ridge in the cylinders and of course any scoring, but I think you know that no brainer.

Look for the lowest mileage vehicle you can. It's probably a best bet when you open it up.
Taxi, limo and police fleet vehicle idle a lot. This should not be to bad of an issue.
Also the least greasy engine. A good sign the seals have held up. Look at the front and rear main areas.