First, 60X80 is not the same proportion as 48X60. 60 is 25% larger than 48, but 80 is 33% larger than 60. So you need to be shooting at 60X75 or 64X80. Get out your calculator and take the sides of each piece and multiply them times 1.25 or 1.33, whichever you choose as your number. So, if you have a 4" square, it's 4 X 1.25=5. A 5X22 strip would be 6 1/4 X 25. That's pretty easy. The problem is when you have 6 1/2 x 1.33 which is 8.64. That's about 8 2/3, which doesn't work for quilting. The 1.33 will be more difficult to work with, as you may end up with some weird dimensions, but just add a consistent % to every piece in both directions and adjust up or down to the nearest 1/4 inch. Be consistent though - either round up or down.
If I used the 1.33 measurement, I think I would assemble the two pieced parts (the squares and the long strips) and measure carefully. Add enough extra to the dark brown sashing around the large bird prints to make it match the width of the pieced parts. You may have to add a little to the tan sashing, so I would cut the sashing after assembling the piecing. Just keep a photo of the first quilt in case you have to check which lines have match up visually to get the look you want. Since you don't have any points to match, you should be able to adjust here and there and still get the proportions you like so much in this quilt. Just keep everything pressed and measure. Remember, measure twice, cut once.
One more thing I do is keep a "measurement" page on the quilt I'm working on. Once I calculate anything, I write it down on that same page so I don't have to keep measuring and calculating. (I had one quilt that I calculated the backing on about three times.)
bkay