sounds like your trying to locate screws into blind holes in the field behind your new panel..
what I've done is start some headless screws in the mounting locations, line up the panel or whatever part goes there, and tap it on the screws to leave witness marks.. drill your through holes there and you should be able to hit the screw hole with a little finagling.. you can even enlarge the holes slightly to increase finagling room as long as you maintain enough grip under the screw head..
I have also made paper or cardboard templates of my hole mounting locations and transferred that to my part.. just have to be careful with orientation.. holes in the old panel may help in this manner..
good luck!
btw, link above didn't work for me but I found the thread.. that post 55 is a doozy
rust repair on the '77...here goes nothin