Cutting up your 'jelly roll race' quilt top into squares can be done, but when the entire jelly roll is already sewn together, it's harder to get square cuts. Of course. that's my opinion, others may and probably will disagree.
One thing you could do with relative ease is cut it into strips vertically, and then run a sashing between each vertical column, border it, sandwich it and quilt. There's still a concern with cutting the columns straight, but at least you don't have to worry about getting it straight horizontally, too. The quilt that you will wind up with is called a 'coin' quilt, so you can look it up online.
Good luck and please post the finished quilt for us to see.