This is not a Turbo 350 ... but the principal is the same ...
You need to know the rear end gear ratio and the height of your rear tires ...

The speedometer gears are color coded ... so you will know the number of teeth on the shaft ... and then adjust. You may have to change both ...

then go here ...

http://www.bgsoflex.com/speedo1.html

Simple really ... to get it dead on ...
Look here ...

http://www.transmissioncenter.net/sp...____va.htm#350