Would using e-body 6 cylinder torsion bars in a 73 challenger with a 340 do any harm?Any pros and cons.

no harm. likely; a slightly softer ride/slight more lean in turns/slight more rise when you nail it (good bennie). If you are replacing em as is return the ride height to the same EXACT position on both sides so as to not disturb the alignment setting (same exact place parked on the pavement (that's my OCD!)/dimention from say LCA bumpers up to their contact point returned to same measurement). EDIT dont forget the jounce and bounce both times to let em settle

For a drag car, the lighter bar helps the front end lift for weight transfer to improve traction for the start - it may help. For the street or in a handling application the lighter bar will increase body roll and reduce cornering ability - it may (probably will) hurt. The answer depends on application.