how do you convert recipes for this many people
It depends on what you want to make. If you have recipes that you use and know how many people it will serve then multiply the recipe to make the amount of servings you want.
Really large batches are difficult to handle and may not cook properly so it may be better to make several amaller batches.
For casseroles I would recommend using disposable baking pans (less clean-up) and making more than one. They may burn on the outside and not be cooked in the middle, Things like lasagna will bake well in bigger batches, but if they are too big you will still have the same problem.
For meats you should allow about four ounces per person. For veggies a little more. For rolls or breads allow about 1 1/2 servings per person (depends on your guests).
It is difficult to answer your question without knowing what you want to make. Hope this helps, if you have your menu planned let us know what you are planning on making and I am sure some of us can be of more help.
when serving a large party of people like forty people always make sure that the meat is easy and simple. it is alwys a good thing to ask the meat market personal what would be a good type of meat to use , they will guide you in the right direction, melody