Tim-- I made a single batch.  When it turned out well, I made a second single batch an hour later.  I'm sure it would be fine to double the recipe and put it in a 13" x 9" baking dish.  I used an 8" x  8" Pyrex baking dish and the bars were none too thick (height-wise).  They'd be pretty thin if a 9" x 9" pan was used.
Matt-- A chiffon cake is on my list of to-do projects.  I was under the impression a chiffon cake was an angel food-type cake, but from the ad I have, I'm mistaken in that notion. The ad calls for using Jello powder in both the cake batter and the 7-minute-style frosting.
John-- You know it!  I'm sure many cooks substituted a less expensive store brand for the Mircale Whip.  I keep the panty stocked with both Hellman's and Miracle Whip, so I used the real deal.
Tom-- Miracle Whip has 3.5 grams of fat per tablespoon. Using 1 cup of it adds 56 grams of fat to the recipe.  No wonder it was moist and creamy, LOL!  There is a mild taste Miracle Whip in the finished product, which is expected as it's the only "seasoning" after salt, pepper, and a tiny bit of minced onion.  Is Celebration Salad the Jello-based salad with cranberries in it?  I have a couple of vintage Jello cookbooks, but no molds, so I need to get online and purchase some.  I've noticed a lot of recipes (Jello and otherwise) from the '40's-'50's use molds.  I want to make at least one Jello monstrosity; probably with tuna, various chopped vegetables, and olive slices in lemon gelatin--with a mayo-based sauce over it, no doubt. I also found one that is basically potato salad encased in Jello.  That might take the checkered flag!
Note:  We are certainly more adventurous/diversified in our seasonings these days.  So many vintage recipes don't extend beyond the basic three: Salt, pepper, onion. I'm used to adding thyme, oregano, basil, rosemary, garlic, bay leaves...