Advanced Formatting for Legal Size Document

I have a legal size form that I use as a merge document.  It consists of an 8.5 x 11 letter with an 8.5 x 3 reply slip at the bottom.  Both the letter and the reply slip print on both sides of the paper.  Both the letter and the reply slip have mailing address as merge fields.

Currently, I have to set up several merge documents to accommodate addresses with three lines, four lines, and five lines to make sure that the top margin of the reply slip stays aligned on the front and back.

Is there a way to set up the document so that the top line of the reply slip (i.e. the lower part of the document) does not move up and down depending on the number of lines in the letter (i.e. the upper part of the document)?