I have bought speedo adapters from Jim Carter in the past. Get your truck running at 50MPH according to a GPS and note what speed your speedometer says you are going. Give that speedo reading at true 50MPH to the folks at Jim Carter. The shop that builds the adapters for them will build a custom adapter for your specific need. The adapter simply screws into the speedo cable hole on the tranny, then the speedo cable screws into the adapter.
http://www.oldchevytrucks.com/cart/product.asp?prodid=SP249&i=9931|||||
Matt
