Have you taken a look at the extender from Wesco Performance for a big rig?
Designed to be used for 3 point seat belts that attach to the seat (usually suspension seats). This extender does NOT click into the buckle end of a seat belt. It is the safest solution to a short belt. This seatbelt extension adds 9 inches to the overall length of seat belts, instead of just to one end (adding length to just one end makes the shoulder harness not centered on the body).
http://wescoperformance.stores.yahoo.net/trsebeex.html?utm_source=googlepepla&utm_medium=adwords&id=62891276046