*Check the marks on the side of the injectors to make sure they're the correct ones for the engine. IIRC, they should read "INP-051", but I can check that if you can read the numbers.
*Try another ECU.
*Try another distributor.
*Check the fuel pressure - if necessary, replace the regulator and/or fuel pump.
*Check the engine coolant temperature sensor - the one that signals the ECU.
*Check the air intake sensor.
*Check the EGR system.

Do you know how much the heads may have been milled?

Are the cams timed correctly?

Have you checked the harmonic balancer timing mark against the mark on the crankshaft sprocket/oil pump to see if they agree? If not, you have a spun balancer..... not too uncommon on these engines with age.

I would suggest pulling some spark lead out of it and see if the pinging quits.

Frank