On a '72 C6 with stock 390, when should the shifting occur? at approximately what speeds?

 

Mine shifts into 2nd at 10-15mph and 3rd at around 25mph. I also just swapped in 4:10's.

I don't know if that's normal but I deal with it, in parking lots i have to keep it in first cause it would go back and forth between 1st and 2nd going around 10mph

 

The more gear(like 4.10) the quicker it will want to shift. Unless you get an adjustable modulator.

It will read higher vacuum due to less throttle needed to get it moving. Thus it will shift sooner(thinking you are going faster then you are due to the gearing).

The whole system works as such. You step on the throttle, this makes a low vacuum signal to the modulator. Once you start to accelerate, the load on the motor decreases, the vacuum increases again and the modulator tells the transmission to shift. Changing the rear gears, changes the way the motor responses and screws up the shift points.