That # kit is for a 73 340 in the book, but I am assuming the flapper you got is say 1-15/16 and the one you want is about 2-5/16 right?
I got a lot of these from a retired tech, and he said they always ordered the kit, and used 1 or two parts, then threw all the stuff in their tool box. I have quite a few from him, and never sold them as kits, because I was never positive all the right parts were there.

Here is a pic of a 3683946 (400) mixed up kit I have. I bet the flapper you have is like the top one, and you want the bottom one, and all the rest looks the same. Measure your bore, and if this bottom one works for you PM me your addy and I will send it.