Wow, I just happened to be looking for a different thread and saw this.
Yes, you can go manual to power. It's not very hard, although the interior of the door is a tight fit if you have large hands.
I will share the same pics that I shared with 64nailhead.
I have forgotten which holes are the ones that the manual regulator bolts to, I think it was probably the ones circled in red, and the electric regulator would bolt to the holes circled in green.
The hole circled in orange was where I attached an insulated clamp to hold the homemade wiring harness that I created for mine, and the blue is where I mounted a junction block to connect my harness to the motor.
I agree with 57handyman also, If you are going to pull the door panel to add power windows, you should consider power door locks while you are at it. For the locks, get the connecting linkages from a donor vehicle.
I had purchased a new window conversion kit (New Relics brand), and also new lock actuators, but used donors from the wrecking yard instead. That worked out good for me because my second G-body vehicle had a left window motor fail about 6-7 months after the conversion on my first G-body, so I just installed the new window motor into it.
When installing the new regulators and door lock actuators, a piece of duct tape helps to hold the short bolts while lining up the bolt holes. I used 1/4in x 20 bolts 1/2in long and nylock nuts.
Hope this helps you out.
Roy, aka "5spdCab"
'78 Caballero, and '79 El Camino