I did this with my SBR Draco-C.
I have a built Md65 rifle on a 74U LLC receiver - I took the stock assembly off of it to make some comparisons and measurements.
I just used masking tape to help me determine where I should begin.
Drilled pilot holes for the underfolder axle.
Once the axle holes were finished I then - kind of crudely - stuck the bit of a hand drill through the right side new axle hole to get starter holes drilled for each of the locking holes. It worked - no damage to the trunnion or anything else. One the starter holes were drilled I dremeled and filed until the holes in the sheet metal receiver matched the holes in the factory installed Draco-C rear trunnion. I put a thick layer of masking tape on the receiver while working on it - more than shown in these pictures.
Patience was the best tool I had in the box during the process.
The only thing I forgot was to drill the two holes in the pistol grip area - those allow the underfolder mechanism pins to be removed if the stock need to be taken apart for some reason. It hasn't needed those holes yet so not a concern.
I finished the work on FBHO's birthday in 2014.