I bought a near new 77 elcamino classic in 78 . It had the stupid air shocks. I split he lines so I could adjust them separately but it was still a fail. Eventually replaced them al together . Even back then the cars were a bit mushy. i added an aftermarket rear sway bar and a heavier front bar. Not much was known about these at the time but it improved the handling a lot. The Ss models had heavier stuff. Even then the 4 gen as a rule handled better than any earlier models. Again the reasons were not well understood. Turns out the earlier one had messed up front geometry.that took a lot of work to fix. The 77 just needed a bit stiffer suspension all around as it was a very heavy car. Of course the weak motor didn’t help any either. Today a good crate motor of any significant power works great. It’s not hard to add a 4 speed overdriv3 trans however in my experience I’d go with the 4L60 or bigger. Yes some floor work is necessary but. At least you won’t be fixing trans every so often. I have a t 350 in my 77 SS NOW WITH 3.55 gears and it works great. The trans is the same construction as the t350 I had in my supercharged 41 Willys street rod. It was still going strong at 35 k. Miles. Other than leaks it worked fine. The ac needs converting to r 134 . It’s probably easier to just rip out the old and replace with new aftermarket unless you really know your way around ac. I had an under dash unit in my Willys and I think in view of the work required I do the same if I were to restore a 77 elcamino. There would be plenty of room for the evaporator but you might have some air box work to do. Most of the sheet metal is still available if you look far enough. No restoration is cheap but these aren’t cheap cars to begin with. Earlier gens are much more expensive. The 4th gen are not as popular as they never were to begin with. But many are starting to see the chassis basic are there and don’t require re engineering. The swivel seats ar almost non esistant now . I asked a local upholstery shop about rebuilding if I could find them and they said forget it. They did not want the job. I’ve bid on one set but but they went for over twice the asking price. There was only one car in the dealer that had them when I got the first 77 so that’s the only new set I’ve ever seen. Almost anything is possible if enough money is involved but I’d surely find something better to do with what it would take to put swivel seats in.Nice looking fourth gen! Check your rear lights. I’ve known a few people who found it handy to be able to turn off the brake and tail lamps at night. (A few may have been in the mountains of Georgia and North Carolina, and may have been in the beverage business).