A body gas tank straps went from side to side through 1967; straps changed to front-to-rear in 1968. The gas tank itself is in the same place, the frame rails are in the same place, springs are in the same place etc., etc.
I have stock Walker replacement tailpipes for a 68-up 340 on my 65 Barracuda (with 69 Barracuda tips). They fit like a glove. Unfortunately, the Walker pipes are NLA. But, basically, whatever will fit a later A will fit the early A, with a slight intrusion from the side-to-side strap which 67 A's also have. So tailpipes shouldn't be any harder to make for the early A's than for the later A's.