I have run my Autolite 46 spark plugs with an early Petronix with the flame thrower coil for about 30,000 miles. The stock 292 is now stumbling a bit with foot on brake at a stop . I figure I'll change the plugs for a start, and then rebuild the later holley 4 barrel if needed. I figure I'll buy another set of Autolite 46 plugs unless someone can strongly recommend a better plug for use with the Petronix. Also, should I gap the plugs wider than stock specs.  The motor is a stock 1960 292 with the truck type heads with the smaller valves. High performance is not the goal. I like a smooth running engine with best possible gas mileage.

You can't expect to get 125,000 miles out of those Autolite 46 copper plugs. If you have been happy with the performance, until now, I would just change the plugs; you should be good with .035-.040 gap. If that doesn't cure the stumble, check your plug wires. If you're still experiencing a stumble, move on to the Pertronix components and carburetor/fuel system, including filter(s). I would also try to buy quality gas from the same station; older stations might have older, contaminated tanks. Off brands might be buying gas from questionable sources. 

I will limit the spark plug gap on carbureted engines to 0.035”.  As ignition systems break down or additional loads are put on the system, it may be necessary to reduce the gap even further.  If you are running gaps larger than this, you can try reducing the gap size and see if that helps.  At 30K miles, you may just need another set of spark plugs.

As Jeff brings up, the quality of the fuel is also a major player.  You don’t mention what grade fuel you are using but in my area there is a ‘by design’ difference in the ethanol content between the 87/89 and the 91/93 octane gasolines.  The 87/89 is typically always right at 10% ethanol while the 91/93 grades are 3-5% ethanol.  I have come across 30% ethanol in fuel that was advertised as being no more than 10% so there are a lot of variables to consider.

Lorena, Texas (South of Waco)