1968 Datsun roadster dies after 15 minutes of run time...running out of ideas to troubleshoot...

If it was run with different pumps and off of a gas can and had the same issues, I would not think fuel.  Easy to tell. 

If it loses spark, it's a spark issue.

I'll be curious to see if it has spark after it dies (that was next up).

They should also pull a hose and see what fuel flow looks after it dies.

The fact it always starts up after an hour-ish sure feels electrical, though.

Edit: "Hooked up in-line fuel psi gauge; starts at 3-3.5 psi (~4 w/electric pump) then falls <1.5 before stalling out"

Is the car running on electric or mechanical fuel pump?

HI all - thanks for all the help!  Some great ideas that helped focus the troubleshooting he was able to do yesterday with a local British car guru.  A few extra details:

• Coil, cap, rotor, and wires were all purchased as part of a package with the electronic ignition distributor, so all are new and should be compatible
• Fuel filter is a new replacement and shouldn't be clogged
• I believe the return line to the tank is for pressure equalization...there wasn't a tank vent to plug as part of the desmog process.
• Unfortunately, he doesn't have the original distributor and/or parts to convert back to points....the original complete distributor went in as a core when he bought the new one.

Troubleshooting updates from yesterday (as best as I can relate not being there):

• Checked vacuum at the manifold to see if there could be a vacuum issue with tank not venting - stable 18#
• Ran a wire directly from battery to coil to eliminate original (ballast?) wiring - no change
• Ignition switch is a bit wonky and needs to be addressed, but not the issue - temporarily bypassed to ensure that wasn't it; no change
• They took temp readings on the exhaust header.  #4 started a bit hotter then leveled out closer to others.
• Put a new plug on a coil wire and fired up when cold and saw good spark.  Disconnected coil wire and grounded to metal...more of a yellow, seemingly weaker spark?
• After letting it run for the usual 15 minutes and stalling out, they checked for spark aaaaaaand....nada.  No spark to be had.
• He did have a spare ignition (amplifier?) module for the EI and swapped that out...no change.
• Pulled the coil wire again and grounded to metal....no spark.

Soooo...it would appear that he may be in the "98% of fuel issues actually being a spark issue" group.  He reached out to the EI distributor guy who responded quickly and is going to go through some additional troubleshooting today or tomorrow, but has already said he'll fix if it's distributor/coil/both.  Given that there was no spark from the coil after it stalled, I'm leaning towards a faulty coil working until it heats up, then working again once cooling off, but should be able to narrow this down.  I'll update once I have more info and/or the final resolution.

Thanks again for all the knowledge sharing...much appreciated!

Easiest way to test the coil heat/cool issue is put an ice pack on the coil after 10 mins and see if it runs longer. I meant to type that in my reply Yesterday.