Is the name Bosch on the plugs or wires? If it is go get AC Delco plugs and a premium set of wires that are not made by Bosch. Bonnevilles simply do not like Bosch at all and have tons of problems with it.
Check the firing order if you removed all the plug wires at once and believe you got them all back in the right place. Seems like something happened during the tune up. Check the gap on the spark plugs and if they the right plugs for the car.
Look for something you may have unhooked and might have forgetten to put back together. The O2 sensor is right under plug 6, check if it got disturbed, broken or unplugged when you changed that plug.
00 Regal GS: All kinds of cool stuff.