It might be helpful if you redrew the top - your scale is way off - 76 x 80 is almost square - and the drawing you have makes it look very rectangular.
And as someone has mentioned - where did you want the motif to 'hit' on the top of this mattress? What are you planning to do about the overhang? Will you make all four sides the same? Will this be a quilt that will go OVER the pillows, or will it be UNDER the pillows?
More questions:
You said you want 2.5 inch white borders - is this cut size or 'finished' size? and you want these borders to be in two inches from the edge of the mattress?