95 Lumina car

blue smoke comming from the exhaust.
recheck you spark plugs.
if that is confirmed, you may have coolant entering the combustion chamber, u need to remove the cylinder head and inspect the head gasket, the engine block, and the cylinder head to locate the source of the leak,

Older cars often suffer more than one cause. Becare not to jump to a wrong conclusion. :(

My guess is that it ran rough prior to the converter being replaced. Typically the top reasons a converter is replaced is because it plugged or damaged from excessive fuel entering the converter, the other reason is it is leaking.-need to know why you replaced it. :?:

Typically blue smoke means burning oil.

Losing coolant may be internal or external. Do you check it at the same time. When cold or hot etc? :?:
Start with a pressure test of the system and verify is isn't leaking externally. Are you using a 50/50 mix?

Recheck spark plug gap and for tightness of wires. Also check the conditions of the plugs, anything abnormal in appearance?- green tint, crusty build up? How were the old ones?

I wouldn't jump into pulling the heads right away, it may not reveal anything. Although it may be the source. Regardless it is an expensive guess.