Hard to say as I don't know what you call rough idle. Your firing order being incorrect would be one reason. Are you sure its a 351w firing order goes counter clockwise on the distributor cap 13726548 the cylinders are numbered facing the engine, closest to radiator back, passenger side 1 - 4, drivers side 5 - 8.

Take number one spark plug out, rotate the crank clockwise with your finger over the spark plug hole and when you feel air blow past your finger, bring the damper to where the pointer is pointing at 6 BTDC, make a mark on the distributor cap base that is inline with the number one cylinder terminal, take the cap off, the rotor should be pointing at the mark you made.