Swap 2.8 V6 TPI to 5.0 305 V8 Carb

I purchased a Firebird/Trans-Am that had this swap done to it. The Speedometer reads fine for the first 40mph, but after that it instantly tops out.

Would this be a cable issue?
Need to recalibrate the speedometer?
Swap out the speedometer for one from a V8?

If its not an electronic speedo, the head (dash unit) is shot. Go to the boneyard and get another one.

Does it matter if the new speedo is from a V6 or V8 dash?