Check power to the coil, 12V at the + side with the key on - this will be fine though best to check. There is a slim chance it will be power to the ignition system (EZK, coil etc, from a fuse). You said the tach died straight away so it sounds like no signal to coil (as that is what drives the tacho). Next is to check if there is a signal getting to the ignition module/igniter/amplifier. Not sure how to do this on the side of the road. It sounds a lot like the ignition module, or maybe (super rare) the EZK failing when hot.
Getting closer though. Also check the spark output from the coil lead, not sure this is easy on a 740 though unplug from the dist cap and rest 1/2 to 1cm from something metal, like a strut top bolt or intake manifold and then crank it over. You will know if there is spark.