Mine seems to be ok for what I use it for, I have 15" wheels/tires too. My speedo is way off but on the hiway it seems to move right along. I don't hit the hiway for very long.
![mmmmm](http://67-72chevytrucks.com/vboard/images/smilies/mmmm.gif)
My 66 has a 71 rear end I'll have to look on the tube or gears to see what it has. I forgot you have a 350 trans.