You shouldnt need anything to modify the speedo cable. It shoudl just go right together with no speed sensor at all. The only thing Id even worry about is thinning out the wiring harness out a bit. You'll have to change a few plugs if you want to have the reverse lights to work. The tranny has its own computer hidden under the console for hte shifter so the 'puter for the engine should be the same.
