I have a 74 260z, LT1, Non-WC T-5.
After recently fixing an overheating problem I took the car out for a spin on some backroads, mostly 3rd and 4th gear. The car ran just fine as usual. I then took it on the highway, 5th gear, 60mph. I went ten exits and exited to turn around. When I tried to downshift into fourth it would not go into any gear, including back into fifth. I sat on the side of the road for a moment before everything was back to normal. When I returned and exited, I let off the gas in fifth as I exited, the car made a loud screaching noise from the transmission, I took it out of gear, same story, would not go into any other gear. The only other problem I ran into was even though the car was in neutral, if I had the clutch pedal let out, it still acted as though it was in gear, or there was some kind of load on the engine. As I slowed down, it caused the car to jerk back and forth, like I was in 5th gear trying to go about ten mph. The car didn't stall, but came close.
Luckily I was a block from the storage facility and managed to get it back into first. The car still does not run smoothly when in first gear, and continues to have problems even when in neutral. I went back this morning and can get the car into first, fifth, and reverse, but no other gears. It's no longer driveable.
I didn't abuse the car at all, just cruised down the highway, so I'm at a bit of a loss for what the problem could be. Trying to be as detailed as possible. Any help is greatly appreciated.