When it was rebuilt did you change valve covers?
Some aftermarket covers don't have the correct oil baffle / breather and allow oil to be pulled past the valve guides under high vacuum conditions.
I had this problem once with an old set of M/T covers and it did exactly as you describe. A picture would help.
Found the pic in your other thread
http://67-72chevytrucks.com/vboard/s...d.php?t=838462