That car is in pretty great shape! The YU was used in several years. All were a 2 barrel low compression 350 so good to go on today's crummy gas. Did you find a date code? Usually it is cast into the top of the block right next to the distributor hole.codes The chain-in-the-tank is a valid procedure. I have a 1983 Yamaha scooter that was allowed to sit for 30 years with gas in it. The carb was a solid lump of junk and needed a new one. But the gas tank I filled with fish tank gravel, solvent, and sloshed it around every day for several weeks. It came out nice and shiny inside. After I installed a new petcock and filter, it is running great. My floors are rusted in the usual spots, the front and rear footwells where water collects. I had an entire front driver side floor pan but I cut it down to replace just the driver floor between the seat mount,rocker, trans tunnel, and firewall. The leftover I used to make a patch for the passenger side. It was a triangle shaped section near the kick panel. The rear footwells I bought re-pops and trimmed those down to replace the rusty area. My welding skills are about the same as Dr. Frankenstein so luckily the floors will never be seen. My old hardtop had no carpets and the noise/heat level was unbearable. Now I usually coat the floors with POR-15 or Rustlok to waterproof them. Then I use the tar underlayments for heat and noise suppression and they quiet it down tremendously. New carpets are not expensive anymore so that goes on top and makes for a nice interior. It is tempting to use bedliner, Lizardskin [lizardskin], and/or Dynamat as a noise/heat suppressor but I would still use carpet to finish it of. Just fix the water leaks so it stays nice. Ask all you need. I have had lots of Firebirds over 50 years and it looks like the folks on this great site have also.