My truck's Seat adjuster cross linkage bar was missing as well, but I was able to find one on the internet from a private individual.

Here is a picture of the piece as I received it:
http://community.webshots.com/photo/109000222/130973072DMItyH

I beleive the PO removed this bar, so he could add extra storage under the seat. Note the metal box which has been welded to the floor under the seat. This box extends down below the floor level.

http://community.webshots.com/photo/109000222/130973040fAkbFe


1954 GMC 1/2-ton Longbed
1971 Pontiac LeMans Sport