Thanks ladies! my customer told me to just use the extra side fabric backing and sew onto the bottom to lengthen it, so I did, it wasn't as terrible as I thought.
I now have another twin quilt for a different customer....laid it out on the floor and it only has 1.5 inches of extra backing on all edges! I immediately told my customer the issue with doing that and its very risky and very well may end up short when I get to the bottom....she is fully aware she needed more backing but said just try my best...so I will but it may be tricky, I have not done a twin size with flannel backing, not sure how flannel acts if it stretches more or not at all etc...so we shall see...I am going to have to keep this one as straight as possible! but if it does come up short, its not my fault, and she is fully aware about it haha...not gonna stress it...