Welcome aboard, nice project. It took me a long time to go with 17” wheels but I’m glad I did. It opens up so many choices for wheels and tires.
Welcome aboard from another Texan in Fort Worth. I currently have three 67 GS400's after recently selling another one I had. Happy to help with any questions, info etc. if I can. Engines were different in 66 from 67 so probably not much help in that area. I really like the black color and recommend you keep it if possible but as you know dark colors attract heat in Texas. Great father and son project.
Buckets have escalated in the past 5 years and are now quite expensive just for a used pair in half way decent shape. Watch for them on ebay or even google them. Any GM bucket seats 64 - 72 should work for you.
The replacement '66 tank offered by Spectra doesn't come with the filler neck. You can have the old one removed from the old tank and put on to the new one. It gets soldered in...maybe a radiator shop can do it. Jim is currently doing this, maybe he can chime in... While you can buy brand new reproduction original style bucket seats they're expensive, about $2k. Check e-bay or Craigslist for used ones, $300-400 is typical but they'll likely need new covers and maybe new foam and springs. 66-72 seats are similar, but only 66 has the metal seat backs and no locking mechanism. Legendary interiors makes the correct covers for '66. They have covers for the bench seat too.
Should be the same https://www.ecklerschevelle.com/chevelle-gas-tank-1964-1967.html Rock auto has them as well. I believe the neck just twist off