This isn't in a chevelle, but i figured theres tons of smart people on here that i'm sure you guys could help. In my 1980 GMC with a 350 small block, I just changed my plugs and gapped them to .045 like it says on the decal under my hood. my truck seemed to be running a little rough before, but when i changed the plugs it seemed to make it sound healthier, but when it was not warmed up i noticed a bit of gas dripping out of my tail pipe like some pistons were missing their spark. when i drove it after changing the plugs it seemed to be lacking quite a bit of power, especially in the lower end. I changed from these http://store.summitracing.com/partde...5&autoview=sku (http://store.summitracing.com/partdetail.asp?autofilter=1&part=NGK-2869&N=700+115&autoview=sku)

to these http://store.summitracing.com/partde...5&autoview=sku (http://store.summitracing.com/partdetail.asp?autofilter=1&part=ACF-R45TS&N=700+115&autoview=sku)

If someone dropped the plugs just once, they are likely to be bad. I would hook up a timing light, and move the inductive pickup to each wire, watching how evenly the light flashes. If you get an erratic one, you have found a possible bad plug.

Also,to add here,if not a bad plug if the plug wires were old and not replaced for plug change when hadling the plug wires changing plugs that can crack/break the interanl carbon core causing an open circut . When that happens no juice is going to the plug/s when the old plug wire/s broke internally resulting in misfire/s poor running motor on non firing cyl's.

Scott

but the thing is it sounds healthier now than before i changed them

Wait until its dark outside.Start the truck up and then go out and open up the hood.Watch the light show.You will find out right away how bad your wires are or where they are not making a good connection.Years ago I took my car back to the dealer for some recall work.When they brought the car back up to me,I thought WTF did they change the plugs or the cam.It sounded really nasty.I got in and tried to drive off and ....no way, it was bucking and missing.They left some of the wires off the cap and some off the plugs.Great Dealer service right.Needless to say unless your wires are new.....Just my.02

Could also be crossed/mixed up firing order if he didnt change 1 plug at a time.
Scott