I've seen identical symptoms on a lot of cars. Including one case where someones kids put dirt and marbles in the tank. So, Same diagnosis here, however, every instance I've seen has been on tanks with an internal sock filter. Dirt would get stirred up and block up the sock, and once the car stopped, it had a chance to settle again. Usually this would happen in 10-15 minutes, only needed a minute or so before the car could start up again, but then would die out again 5-10 min after that. The S30's dont have an internal sock filter, but it could very well be that a big chunk of rust or dirt(most likely rust) got loosened up and is blocking the feed line that's inside the actual tank on the 260. If it were in the line or filter already..... odds are it would probably stay clogged or clog again within a few seconds to a minute. There's a chance some crud made it's way past the filter and got into the actual carbs, but I'd look more toward the tank for the problem first. I've had 2 Z's with super rusty tanks. Both of which did not allow any flow out of the tank itself.
I've sold and used SeaFoam on dozens of different types of cars for a number of years. It wont affect the carbs themselves in a negative way. But it can stir up a lot of dirt and grime gremlins... I personally use the Brake booster line method rather than putting it in the tank 99% of the time mainly because I'm concerned with the oil/carbon build up in the intake manifold and combustion chamber. It's a lot more effective on the carbon build up that way. (sorta like the old water through the carb trick)
S30 tanks are great in that they have a drain plug, I'd suggest getting a container, pullin the plug. See what comes out first off. Once it's empty, you can take the fuel level sender out and take a better look inside without having to drop the tank. That's of course if you're not sure that you actually want to drop the tank.