After sitting for 26 years, I'd be surprised if the transmission works at all. Seals do harden over time and are not supple and flexible. Also valves in the valve body can become stuck and hung up. Servos can also be stuck as well as the valve in the governor.
If you've already tried adjusting the TV Cable, the very next thing to do is, remove the governor on the side of the transmission. Make sure the valve is moving freely within the governor bore and the weights aren't stuck, and the gear isn't stripped. Feel free to clean that governor with carb cleaner spray to remove any varnish or sludge. Reinstall the governor and see what happens.
If it still doesn't work....
I'll give my speech again. I am not one for liking additives in engines and transmissions. But some do work. DO NOT be hard on this transmission or you will kill any chance of bringing it back to life if it's even possible. You will need to get a GOOD transmission conditioner additive in hopes of getting the stuck valves and seals to become free again. Transmission Conditioner additives do not FIX ailing transmissions. However, for a transmission that has been sitting for a long period of time where the seals have hardened or become stuck in the drums, a good trans conditioner will help soften the seals and help them become more pliable and supple again. Do NOT expect a miracle to where you just pour it in and it works. It will take some time. It will also take some heat. DO NOT be doing any jack rabbit take offs. Because you don't know of the TV cable adjustment which also raises fluid pressure to hold clutch packs tighter, you can burn up the clutches quickly if the fluid pressure does not raise. You will probably need to let it idle for quite some time allowing the radiator coolant to warm the trans fluid. Run the shifter up and down through all the gears every so often allowing it to sit in each position for a minute or 2. This will open up that circuit and let the warm fluid and additive work thoughout the entire transmission. Take it for a drive in your neighborhood on the residential streets even though it will stay in 1st gear. It may eventually shift to 2nd. You can also use manual 1st gear and move it to manually to 2nd gear. That might create a shift in itself by doing that. 2nd in manually selected 2nd is achieved in a different way than 2nd gear in D position and so is 1st for that matter. Just read the directions on the bottle of additive and use the correct amount. If it still doesn't work, maybe add a bit more additive. No need to go overkill with the additive. It'll either work or it won't, and if you get to much additive in the transmission, it can actually do more harm than good. Just keep in mind, it's going to take a bit to get the seals warmed up and allow the additive to soften the seals. It won't happen immediately.
Also don't forget, if the TV cable is adjusted with to much pull, that will also keep it from shifting. If and when you get the transmission shifting, then try the TV cable adjustment. again for a proper adjustment.
