I used the Cliffords when I replaced the old 3:2 dual headers on my E100 p'up 170ci. The difference in torque and throttle response was quite evident. The 6:1 is a great way to go.

Though I haven't put headers on my '66 Falcon van, it has a 240 in it which is the same block as a 300 so you should be able to make that work. Give them a call they will KNOW. No guessing required.