I am in the process of replacing the entire floor of my 72 right now. I worked on getiing it out this past weekend and this next one we'll put the new one back in. Let me tell you it is a MAJOR job but I think that it's well worth doing. It took us about 5 hours to actually cut the floor out and once we get all of the pieces cut and welded together I think we'll have it all back in in about the same time. I did leave about an inch all the way around and I'm replacing the crossmembers with box tubing that will eventually become an airtank, but that's a whole other project. Just be sure to measure everything very carefully and take your time with it. You'll be glad you did. I know that I will be.
By the way the diamond plate steel that I bought was 82.00 for a 4'x10'x.125" thick sheet. A lot heavier-duty than the replacement steel and you can weld or seal the length of the cross-channels, so the rust doesn't build back up in there, too
HTH,
Buddy
I've got my dream truck and it's a 72 K-5 that is getting there!!