360 and 727 question //help

i have a 78 360 with 727 trany , the motor is external balance , i have a tci breakaway stall converter i want to install to this setup, being external balance do i need to change the flex plate or buy weights to weld to converter ,i notice factory converter have a weigh add to it. if i need to chang flex plate anyone know part number i need ? any help would be greatfull