Goood for you for checking it out in person!
1) The deck lids are interchangeable from 70 to 81.
2) The 73 nose is very different internally. That was the first year of the 5 mph impact requirements and there are at least 40 pounds of extra brackets and mounting hardware in there. The beak is also about an inch longer than the 70-72 urethane nose. They look similar but are not correct and may not bolt up unless you use the 73 and up radiator support which as we know would look incorrect.
Here's the 70-72 nose. Note there are only 2 steel "L" brackets that hold the nose to the frame. Also note that the two grills have a honeycomb pattern in 72 and a small mesh pattern in 70 and a still different pattern in 71. 73 was entirely different with a large egg shape mesh pattern and was shallower in depth.
Here's the 73 nose with a bunch more hunks of metal.
3) If the left and right tanks are ok the radiator can be recored. That 4 speed radiator will not have any tranny cooler fittings. That is the telltale that it's a four speed radiator.
4) I guarantee that there will be extensive rust under the left and right lower corners of the windshield. That is the bad spot on the 2nd gen Firebirds and Camaros. If you look into the lower corners of the windshield next to the A-pillar you will probably see a bunch of scale - a real bad sign.
5) They do sell nice, accurate full floors for these cars. You'd probably be better off replacing the entire floor rather than welding piecemeal if there is rust in all 4 seating positions.
6) Trunk floor rust is common. They do also make good full trunk floors. I'd buy the full floor and use what you need and stay away from the smaller 1/2 floors, they're not as accurate.