Under Cab Issue

['67fleet]

I recently got a 67 chevy and I am just starting to find out what needs to be done to make it safe and reliable. Right now my biggest concern is the rust damage to my cab's support frame (There are pictures of what I'm talking about)
I was planning on using this as a daily driver for a little while before I took it off the road to do any major work but I'm not sure it is safe to drive this way. How would I fix this? All four corners have rusted away but everything else seems intact (you can see in the first picture what the rest of the under carriage and the actual frame looks like).

Cheers