You could always start with your option 2) see how it looks at the point where you have the 06 floor and firewall hanging out and exposed; then you will have a better idea as to whether or not it is the way to move forward; you can do this and make a decision before doing any cutting on the 48.
If it does not look like it is the right direction at that point you can always fall back on your option 1).
I have never carried out option 2) myself. But i have seen a blend of new & old built in that fashion.
