If you are talking about the threaded portion of the spark plug, you just can't use a shorter plug. The threaded portion of the plug length has to match the threaded portion of the spark plug hole in the head. If the plug is too short then you will have exposed threads from the spark plug hole. If the plugs are too long then you will have the threads from the spark plug exposed. Exposed threads in the combustion chamber will cause pre-ignition. That being said you might be able to index your spark plugs so they clear. You also might be able to find spark plugs with shorter electrodes. Or something maybe a surface gap plug like this:
https://www.summitracing.com/parts/n...BoCHSgQAvD_BwE