Hi,
The C-6 should have a kickdown linkage from the carb to a 3" lever on the driver's side of the trans. This lever controls fluid pressure and shifts at wide open throttle [ WOT ] .
If you don't have this then you need to get it before doing anything else.
If you have this ,I would take it to a good trans shop and get it adusted and change the springs in the governer so that it shifts higher.
Perry.
