I know the phrase stuff as well. Don't you also repeat the ad'libs in reverse order at the end of each verse? I'm trying to remember what they all are:

She'll be comin'... "?" (it might have been Toot, toot)
We'll all go out... "Hi Babe!; Toot, toot"
She'll be drivin' six white horses ... "Whoa, Buck (back?); Hi, Babe; Toot, toot"
We'll kill the old ... "chop, chop; Whoa, Buck; &tc."
We will all have chicken... "yum, yum; chop, chop; "
I will have to sleep with Grandma ... "?" (We didn't do these last 2)
She will wear her red pajamas when she comes.