I've done this swap. The reason for swapping to the later crossmember is because the front end will not align to factory specs unless you use it. And it isn't even a big deal, 4-6 bolt unbolts it from the donating truck, the same amount bolts it to the 64 frame. You don't even have to unbolt the lower control arms, just bolt it in. Press in new ball joints (spec for the later truck) and bolt on the spindles. Easy as that. Of course you will have to take the brake lines, proportioning valve, and master cylinder (& booster) from a 67-72 truck (fits without modifications) or a 73-79 truck (have to modify a bit).
