Can this happen? I have a 74 d100 and it randomly stops running and won't start for awhile. I changed coil and ECU with known good ones but was wondering about the starter relay since it has a spot for ignition.

Off the top of my head, the ignition wire at the relay is jut to bypass the ballast resistor when cranking. Unless it shorts out somewhere I don't see it causing a stall situation.

Assuming the problem persists after changing the ECU and coil, I'd be suspect of the pickup coil in the distributor.

Of course 50 year old wires are always suspect.

Agreed, and the ballast is bypassed in the start position so that eliminates it.
I would check the see if you're are getting spark when the problem occurs. If not, verify grounds even if you've already done so. I have in th epast run a small braided ground strap between the ECU and engine block making sure both surfaces are clean of paint etc.
Using a brass or non magnetic feeler gauge, check the reluctor gap to pick up coil, should be .008-.010" .
usually a good idea to check it in a couple of places. Then try the pickup coil.
Keep us posted

Make sure your voltage regulator has a good ground.

I have had to sand paint off the back where the screw holes are to get a good body ground. (I used dialectric grease on the bare metal)

I fought that die suddenly / no restart issue for months with one car, no issues years later after ensuring a good ground.

Some troubleshooting rather than ransom parts swapping might help.

Are you even getting voltage to the coil when this happens?

I had one that would randomly die and immediately restart until it died again. Could be 5 or 50 miles One of the pickup wires inside the distributor had broken internally from the vacuum advance moving. Found it while trying to stretch the wires. One stretched the other didn't

Every early one Ive had had a problem with the amp meter.Seems they burn and would let the vehivle die.Then they would restart.I pulled every one and used a small screw and nut then taped the 2 wires together eliminating the amp meter.Then install a volt meter

A good volt Ohm meter and knowing how to use it are your friend on every electrical problems on our vehicles.

No, the starter relay will not cause the car to die.The "I" terminal is the start signal from the ignition switch.

I asked because a guy on YouTube said ignition relay was shorting out internally and caused his truck to die. Getting voltage to the coil. Of course I checked that. Problem is intermittent making typical voltage checks difficult.

That’s just in lates 70 setup, where the ballast bypass comes from starter relay instead the ign switch like previous years. But by 74 the ballast bypass still comes straight from ign switch.

So by 74 there is no any kind of connection between starter relay and ign system where a short could make anything on ign system

Not saying this is the issue but we chased a random / topend misfire on my wifes '68 Barracuda for a while, it would also cut out going round corners from time to time. Turned out to be the key ign switch.

When this happened to me it was the bulkhead connector. Pull it off and look for corroded, melted or burnt terminals. As Cab said a volt meter or even a 12v test light can be used to track down problems like this BEFORE you start swapping or buying parts.

hang a temporary light bulb inside somewhere visible and see what it does when the issue occurs

Some times after the pickup coil gets hot, being inside distributor, it will lose conductivity.
After cools somewhat, reestablishes conduct.
Bad wires, or an internal connection, possible.
Put ohmmeter on plug & wiggle around to see.
Or just replaces with new.

The starter relay switch is a safety device intended to not let anyone engage the starter without the transmission being in in park or neutral. It is not involved in the ignition run circuit with the ignition switch in the run circuit, it is involved in the start circuit

That's neutral safety switch.

For many years Dodge trucks did take the resistor bypass from the starter relay. Possibly not in a 74 though per one poster here.I don't have a diagram for that year.

Neutral safety switch supplies the ground for the starter relay. which has to be grounded to work therefore the safety device part.With a standard trans you can just ground the starter relay.

I had the same problem with my 73 Challenger. Car would run for 30 minutes or so, then die. Had to wait about 15 minutes before it would restart. Turns out it was the ignition switch.

The neutral safety switch, depending the car year and model will be either in the tranny or on the steering collum, the starter relay that activates the starter solenoid from the ignition switch is usually mounted on the firewall