The Early years of K2 GM platform trucks had this same issue.
Dealer attempted fix consisted of plastic clips that kept the seat bracket from shifting to the
side causing the clunk.
My fix was a rubber hose and clamp!
Sounds like the T1 trucks are plagued by some of the same problems as the K2s
Along with creaky rear leaf springs and other.
Here is a 70 page thread on the seat popping noise issue.
https://www.gm-trucks.com/forums/topic/154219-seat-movement-issues/page/70/#comments