A couple more thoughts. Are you sure you want it to completely fit the bed? That means you'll have overhang at the top as well as sides and bottom. I usually make my quilts a bit shorter when there's a symmetrical pattern I want to display, so that the pillows don't interfere with the design. So what about making your finished quilt to the borders, 72x72? Then there would be 6" for the pillows at the top. And the numbers are nicer to work with
Cut your rectangle down a bit, to 52x60. Aiming for 72x72 - add 10" borders on each side(6 blocks, 10" finished, your inside finished length for the on-point blocks is 7 1/8"). Then add 6" borders top and bottom (12 blocks, 6" finished, on-point blocks are 4 1/2" finished).
Or, would you consider turning the rectangle on its side? Then you'd have 60x52, with a finished size of 72x78. You could put 6" borders on the sides, and 13" borders top and bottom (or, 2 6.5" rows for better symmetry). Won't work if the rectangle is directional, of course.
Editted to add - I just noticed a problem in your calculations. You want a 2" border all around, but you only subtracted 2" from the bed size. You should subtract 4" (2" border on each side of the top). And since I apparently cant' read I used the wrong numbers anyway. You should be trying to achieve a 72x76" top from your initial calculations Then, 2" on all 4 sides makes it 76x80 finished. Of course, 76=19*4, which is an even worse number to work with.