I just swapped a 350 into my camaro attatched to a 700R4. I previosly had a 2.8L V6 in it. I swapped the tailshaft on the V6 700R4 to the V8 700R4 to utilize the torque arm mount. Could this be why my speedometer is reading a higher speed than what I am actually driving at or is a V6 700R4 speedometer cable different than the V8 700R4 speedometer cable?

 

It's not actually a problem with the speedometer cable but the gears that turn it in the tranny tailhousing.
When you swapped in the V8 tranny tail, you put it in with a different speedometer gear combination. I'd recommend removing the speedo gear housing on the old tailhousing and the new tailhousing and looking at the gears. Hopefully you have the same drive gear and would need to swap only the driven gear.

Get back to us with what you find.
Lou