noise problems at high rpm

hello!
I have problems with my engine, a mopar 408. In first gear it works up to 6000rpm then it starts popping in the exhaust, in 2nd gear it will go to 5300rpm then it starts popping. First i thought it was the the lifters that pumped up so the valves were floating, but it was not. Then i saw that the spark plugs where a bit whiter than it should be so I changed jets in the carburetor, didn´t work. So now I think it is a electrical problem. Can it be the coil that is broken, although the sparkplugs looks good? or anything else??