It sounds like your quilt is already quilted, so it's too late for squaring up before quilting. Your best option is to try blocking the quilt to see if you can even it up. I think that will probably work fine, since you are not off that much. However, if that doesn't work, then you have two options. One is to go ahead and slice off some of the side that's too long and accept the fact that the borders will not be exactly even. The other is to leave the quilt as it is. With a bed quilt that large, you won't ever notice that one side is 1" too long, and after a few washings it may even itself out anyway. Don't beat yourself up over it - NO ONE ELSE WILL EVER KNOW! (Because we're not telling.)