15 inches of vac at idle
15" vacuum does not seem abnormal for a 268/272 duration cam...??
Where do you see he has 15" of vacuum? Are you looking at the difference between cylinders on the compression test? The test numbers show 5 cylinders to be pretty low which sort of indicates a 'gross' problem.
A good way to check to see if the rings are not sealing is to do the compression test 'dry' like mentioned above and get your number. Then, squirt some oil down the hole and do the test again. If the numbers on the gauge increase, then its the rings. If not, then you have other problems.
If you had a blown head gasket, it likely would manifest itself by overheating and white smoke out the tailpipe. You can do a rudimentary check for that when it's cold. Take the rad cap off and start the car. Let it warm up a little and look down the radiator filler neck for bubbles. If there are bubbles, you likely have a head gasket leak but it's
not a definitive test. You need to confirm with a coolant pressure test at least or with a chemical head gasket test.
If possible, a leak down test would be best here. You would be able to pin point the source of the lost cylinder pressure much more easily.
In lieu of that, pull some plugs and take some pics and post them here. Shine a light down the spark plug hole and look for damage. Check your oil for anything odd. Basic stuff first.
You didn't mention any timing stuff either. Do you have that handy?