Is is typical for the floor pans to be Swiss cheese and the frame cross members quite respectable?
Quite typical as long as the water has a chance to run off. Floor pans have a tendency to hold water, until of course they are rusted badly enough to have all of those pin holes. Once the pin holes rust rapidly takes over.
I have a SIV GT BW35 Alpine that was given to me because it was thought to be beyond recovery.
It was part of a deal where I purchased my Colour Code 70 ALpine. I had to drag the BW35 off in order to get the other one.
I spent some time going over it to see what if any thing could be salvaged....Glad I did. Put in storage 25 years, now it is one of my prize Alpines.
The floor pans and outside body panels were all rust! I was amazed at how the undercarriage was preserved.... NO RUST ANY WHERE. Learned that the PO had covered the BW35 Alpine for years with tin sheets that kept water away from any further damage to the inside.
I removed the floor pans and saw what you guys are speaking of. The Crossmember was in excellent condition. Amazing!
I did discover areas in and around the firewall where water damage and foreign matter could eventually penetrate and start rust inside the frame.
I highly recommend you cut some drain holes approximately 1" to 1 1/4" in diameter in the bottom of the "X" frame to allow drainage of water and other foreign matter. Look for the lowest point in the crossmember/frame for the best place for the holes.
I have had several Alpines where the X frame has been totally destroyed from rust because of foreign matter getting in side of the frame. Seems like the most probable place is right near and under the "A" Post.
I have some pictures that depict the area most likely to get damaged.