If my heads aren't bad, what is causing white exhaust smoke?

White smoke is coolant. Does it smell sweet? how did you run a pressure test? was the engine up to temperature? Cracked heads don't always show a leak with a pressure test (assuming you meant a simple compression tester) did you do a cylinder leak down test? does coolant cross through your intake from head to head? there could be a leak there. Where ever it is, you have one.

White smoke means that water or coolant has reached the combustion chamber. Run a compression test on all cylinders to see if any fall out of the 10% range. 5 % would be ideal but let us move on. If any cylinders fall out of that 10% range, a second test can help to define where it is going. A cylinder leak down test along with a bit of know how can lead you to the fault.

Essentially, you connect an air hose to the compression tester and examine the 4 points around the vehicle.

As long as the engine is at tdc compression for that cylinder, the air should be stuck in that compression cylinder but since you believe that you have a leak check for the leak. Pull the oil cap off and listen. Any hiss would mean the compression rings have failed. The tail pipe, any noise means the exhaust valve is not seating or damaged. Air filter housing, a noise would condemn the exhaust valve. Pull the radiator cap off and look into the coolant. If you see bubbles or motion caused NY the bubbles posing, then you cam have a cracked head or block along with a blown head gasket. After you have defined the fault, the fun starts sometimes with finding the fault..

white smoke is coolant - if it isn't heads or gaskets, then it's something drastic like cracked bores etc.

but despite your tests, I'm guessing it's still a bad gasket......'specially since it's a Buick v6. run without pressure in cooling system: see if smoke decreases or you get bubbles in radiator.

ADDED: I think "Bandit" is onto something with intake gasket theory - I'd forgotten that the Buick V6s were sort of notorious for that - specially if you are lucky and have one with the plastic intake manifold !

Try a simple test. Have someone follow you as you drive 55 mph. Lift your foot completely off the throttle. If who ever follows you sees white exhaust smoke, the clearance between the valve stems and guides is too great. New valve guides will put your motor back is business.

possible oil? have a mechanic inspect the cat converter...check for oil leaks or for coolant loss in the reservoir and radiator...other than that...it could be a head gasket...

The thing is 25 years old. Valve guides.

Oil. A flaw in a gasket or a crack in the body.

intake gasket ! if the intake gasket is leaking it can put antifreeze into the combustion chamber too !