[Buttercup]

Bought a 66 390 from the original owner, and it ran well. It had only been driven 500 miles per year for the last 15 years and lived in a heated garage. Within a few days and about fifty miles of driving it began to run worse and worse, and the oil is a milky light brown. Car has had a tuneup and carb rebuild. The way it runs suggests vacuum leaks to me. I'm leaning towards replacing intake manifold gaskets, but is there any way to determine if that is the problem? Could it be head gaskets as well?

Thanks in advance!