Let's recap:
You cut a 6.5" border that you want to attach to the quilt center, but you're a bit short on the length. So you're going to cut it down X" (X being the length of the quilt top).
If you're using the same width for the top and bottom border (and there would be no reason not to unless you're a masochist), then the blocks you're going to need for the corners have to be a 6.5" square unfinished.
So your pinwheel block would be made up of FOUR subunits, each a 3.5" unfinished HST blocks.
To make life easy, I would cut eight 4.5" squares each of the 2 fabrics you're going to be using for the pinwheel,
make the HST (any method), and trim down to 3.5".
Then sew four of the HST's together into a 4-patch.
And, voila, a 6.5" block. Make 4.
Attach one to each end of your top and bottom borders (the center portion being the width (unfinished) of your quilt center).
Attached to your quilt top.
I think the math is write unless I made a typo.
Clear as mud?