See the picture at the link, this shows where it is safe to drill a hole and where it’s not.
If going through your effort, I open up the hole and weld in a stiffener that I tap for a plug. To keep the pan from wrapping I bolt it to a flat piece of at least 3/4 plywood or better two layers glued together so it can’t warp when heat is applied.
A deep pan is a big help to these transmissions along with external oil coolers as these overdrive trannies run hot, especially if this is a working truck, that includes working hard for having fun.
Be sure to seal the dipstick tube really well, the GM anti-drain back valve on the converter is notorious for leaking which overfills the pan such that if the filler tube and or the TV cable are not well sealed a lot of oil will get out. Permatex ATF, RTV sealer part number 81180 is made specifically for ATF sealing. Just incase you haven’t noticed ATF is a fantastic penetrating oil, if there is a way out it will find it. Additionally 81180 is formulated to resist the chemistry of ATF that dissolves most anything it contacts given a little heat and time.
Be careful if you screw around with the cooler line fittings the threads in the case are straight pipe thread not taper pipe thread, using standard plumbing fittings will crack the case.
Bogie