Use the pulleys from the same engine that you pull the timing chain cover, water pump, and alternator/PS brackets from. (You do know all that needs to be changed, right?)
In other words, if you use pre-'70 parts (iron water pump- driver's side inlet- and accessory drives), use pre-'70 pulleys. If you use '70 and later parts (aluminum water pump- pass. side inlet- and accessory drives) use the '70 and later pulleys. Easy-peasy.
Make sure your 5.9 has the 2 piece harmonic balancer and pulley- later magnums used a one-piece balancer and pulley.
If you use the early year stuff, you'll have to file one hole (the offset one) on the crank pulley to make it fit the Magnum balancer. 5 minute job.
