One thing I could add to NM51's fine advise is to take two or three bolts of the same thread as the pan bolts, cut the heads off, thread them into holes on each side of the opening. Use them as guides to line the pan up as you install it. Once you get the rest of the bolts in place you can unscrew the headless bolts & put the regular bolts in. Make sure they are long enough for you to grab after the pan is on.
