Your last paragraph says it all.
I also would weld nuts on the latches as it will be nearly impossible to get a wrench up in there without cutting an ugly hole.
I TIG welded nuts on mine the very first time I took them out. there are 3 flat head hes socket screws that hold each latch on. You may also find that hooking up the 'down' or actuating rod will help installing the latches. I did this before instaqlling the PW. I made small right angle bellcranks mounted on 1/4-20 screws and tucked the actuating rods in close to the inside of the door. These come off at the bellcrank to install the PW.
good luck on a tough project.
Wait untill you get to the wipers!!!!