im in the process of doing this now. Well, ive just put the 140mph speedo in and it only reads speeds correctly up tp 60 without that black thing on the needle (its a weight), and with the weight on , when I was stopped completely, it redd 10mph. I dunno, i was doing this in my 4 cylinder. But since you have a 5 liter tranny, it might work just fine.
its not really that hard to get out. I hated disconnecting the headlight and hazard lights though, pain in the ass gettin them back in, and theres about15 screws you have to take out. But like i said, its not that hard, just a lil frustrating.