My understanding is that the foam for 69/70 buckets is different from the 71/72 foam. It may be only the foam for the backs that differs. I have no idea if the 69/70 and 71/72 frames themselves are different. As Dave said above, the Blazer and truck buckets are the same except for the back on the Blazer buckets and the passenger side mounting brackets.
Here's a link to a previous thread that may help.
http://67-72chevytrucks.com/vboard/s...d.php?t=437963