The real question is, "Will my 1982 corner castings fit over a 1971 hull pylon?" The dimension between the pylons is irrelevent (to a point) since the side bars "float" in the corner castings.

The answer is "probably." But you won't really know until you try.

The corner castings were changed slightly in the early 80's to better fit the pylons and to prevent cracking. Odds are the '71 pylons are so worn that they'll be really loose in the castings, no matter what year your castings are.