I believe the 2 factors involved are the resistance rating of the new turnsignals and the total amount of circuit impedance/resistance with which the stock flasher relay is designed to operate. If you have access to a multimeter/voltmeter, you can measure the impedance (in ohms) of the stock signals that you removed and the new, LED signals, the difference between the 2 would give you the value for proper resistors.
OR, you could just call the LED signal manufacturer and see if they can tell you.
OR, you could just call the LED signal manufacturer and see if they can tell you.