Car Quits At Speed :( stressed

X3 on fuel delivery. Vapor locked fuel line? Steel line too close to engine block or exhaust pipe? It will "boil" the fuel into vapor. The real time to diagnose is exactly when it stops. Immediately pull the air cleaner and pull back on the carb linkage and see if fuel "squirts" into the carb. If not, try using a (little bit) of engine starting fluid when it quits. If it's a fuel problem it will start or try to start right back up.

If its the ecu it will not. Change the ballast resister and ecu when it quits. You could even mount an extra ecu on on the firewall and just switch the plug on the side of the road.

A friend of mine took his show car to the Rod Run at Daytona and had a problem like yours. He replaced the ecu twice and it would start, then next time NOT start. A bad ground on the ECU to firewall might do it. My friend got POed and replaced it ALL. ECU, Ballast resistor and Dizzy. It's already been suggested but an intermittent problem in the distributor pickup will stop it dead in it's tracks when it shorts out, and then work fine later. Remember, the vacuum advance keeps that pickup moving around inside the dizzy and it could get up against the shaft if not adjusted properly.

Here's another idea on the fuel end: something large floating in the fuel tank! I once heard of a practical joke where you put condoms in a persons gas tank! They float around, then get sucked into the fuel pickup. When that happens, fuel delivery stops, engine dies! After the engine stops, suction stops, condoms fall back into tank and car starts. The "joke" was that it's almost impossible to diagnose.
Any large item in the tank could do that.