Excessive oil consumption

Do a compression check on the engine. If the compression shows low on one or more cylinders could mean a blown head gasket or the rings a shot.
In that case the engine should be rebuilt, when you did the valve stem seals did you have the heads off?

How many miles on the engine?

On the older 289 and 302's, (identical engines except for basically the piston stroke) the use of a quart of oil every 1500 miles is considered normal. Even a quart every 1000 miles isn't something to worry about. That's with umbrella type valve seals and soft iron rings. So the reason I ask the engine mileage, is because a worn out 289 will typically use a quart or more every 500 miles.

Check your PCV system.
Check for any vacuum leaks, you could have a leaking intake gasket that is letting oil into the intake runners.
Do a leak down test of all your cylinders. That will tell you whether or not your rings are badly worn, broken, etc.
Even though you replaced the stem seals, the valve guides themselves might be severely worn. The area below each intake valve is a very high vacuum area, and will pull oil right past the seals if the guides are severly worn.