Hmm, curious. The holes in one side of the crossmember are 1/2", as are the holes and bracing tubes in the frame. It's the holes in the other side of the crossmember that are a little smaller.
67 is probably factory 1/2 inch and the 71 cross member is 3/8. You will have to either pull the cross member out and measure or just drill them out to 1/2 inch. I believe they went to 3/8 in 1970