This is news to me, as I have allways swapped pulleys from year to year off cast crank 318s, right onto steel crank 340s.... Maybe that 73 340 balance weight is in the way of the pulley? never thought of that....
Not only is the bolt pattern of the crank pulleys different, which can be overcome by grinding out one hole, but the depth is different and will require the corresponding brackets and pulleys for an A/C car with a cast crank!