Car has rough idle

if it wasn't idling rough before the spark plugs but is now, there is either an issue with the sparks or there is a compatibility issue.

It could also be the battery or loose connections, I'd reseat the HT leads and ensure a solid connection, I'm unsure if that car has coil packs, but it could be those are on their way out as well, so replace those too, newer plugs pull more voltage, which in turn causes the coil packs to need to generate more power and if they aren't up to it.... You get rough idling.

Regards,
Alex

Can you describe what you mean by "rough idle" in a bit more detail?

How long after stop/restart does it return?

How old is the vehicle, how many miles?

Did you have a dealer pull codes from the computer to see if there is any diagnostic info there?

How long has this been happening, might just be a bad tank of gas.  You might try a can of gas additive designed to help clean out any water in the tank in case that happened.

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»bp

If the problem didn't occur until the plugs were changed, they may not have been gapped properly.

There is a screw around the carburetor, etc. that can be adjusted to affect idle. When my vehicle is idling rough when sitting at a red light, I look into that adjustment. That is a remote possibility in your case.

There are many reasons for rough idling, but these days most of them will turn on the MIL and generate trouble codes which are stored in the ECU's memory for retrieval by fault code readers and other diagnostic equipment. These codes often (but not always!) give an indication of the problem's origin.
If the car is has fuel injection and a catalytic converter there will be no adjustment screws for tweaking the idle, as carburettors are now so last century and haven't been standard equipment since catalytic converters became a legal requirement in most markets. Compared to managed fuel injection systems carburettors are rather crude and imprecise, and contain far too many moving parts to maintain accurate fuel metering.

Coil packs (or pencil coils, or coil-on-plug...) are a likely source of the misfire, as they are electrical/electronic components operating in a very hostile environment; heat and vibration are killers of such equipment.

I would correct a statement made earlier; new plugs require less voltage to fire them, but the available energy produces fatter sparks of longer duration because less energy is used in overcoming the higher resistance and larger plug gaps of plugs that have seen long service.