I heard from one guy that u can swap a 140 mph speedo into a v6 85 mph one and will work right without problems, other guy said you need a yellow speedo box for it to work.

Are any of the two right?

 

Depends which year, but I'd think if both speedometers were the same VSS style, it'd bolt right in. The two styles were speedo-mounted VSS, and trans-tailshaft-mounted VSS. The trans-tailshaft-mounted VSS had a yellow VSS buffer box located next to the computer. And, of course, the speedo-mounted one hung the VSS+buffer off the back of the speedo.

So if your speedo has the VSS in the back of the speedo, you can easily swap in another speedo with the VSS in the back.

If the VSS styles were different, I'm sure it still could be made to work, but I'm not sure of what wiring would be involved to convert between VSS types.

So I think both guys were right