I've actually never seen the SD to SEQ kit, but the piggy back idea sounds like an easy installation. And here I am repinning connectors

Looked at your other thread. And the EEC has multiple grounds...

Pin 20 is the case ground
Pin 40 is the power ground
Pin 60 is the second power ground

The bank-firing speed density computer (your old one) has the injectors wired in two banks, fired off pins 58 and 59. On a 5.8L engine they are grouped like so:

INJ 1 (58) #1, #4, #5, #8
INJ 2 (59) #2, #3, #6,#7

I confirmed the EEC "grounds" the INJ1 & 2 pins to fire the injectors, and the other side off all eight injectors receive +12V power from the EEC power relay.

Both power grounds (40 & 60) are connected together, to black/white wires that are screwed down on the right front of the engine compartment on the fender apron.

The mustang mass air stuff is different as you know...

Pin 12 - Inj 3
Pin 13 - Inj 4
Pin 14 - Inj 5
Pin 15 - Inj 6
Pin 42 - Inj 7
Pin 52 - Inj 8
Pin 58 - Inj 1
Pin 59 - Inj 2

Like with the trucks, they share a common power point (the red wire of each injector connector) which tie together, and go to the EEC relay.

6, 8, 2, 1 are not firing? I looked for overlapping functionality, but didn't see anything obvious. Pin 12 would be connected to the 4x4 enable if you have an E40D. Pin 42 would be connected to the TOT line of the E40D. And 52 connects to SS1 of E40D. This would be in parallel to injectors 3, 7 and 8, but your having problems with 6,2,1 which has no overlap between the wiring diagrams.

I do not understand the 3-wires per injector thing... injectors have two wires.

What is the chance that you have access to a code scanner of any kind... maybe the EEC (A9x?) can help us out here? I don't see any obvious wiring overlaps, all the injectors should be tied to a common point to the EEC relay, and I assumed you checked to verify that the red wire on each injector has power whenever the EEC relay is closed.

Did you test light the injector connectors yet?

You can also try something "insane" like swapping a "bad" injector for a "good" injector. Swap 7 and 8, since 8's dead and 7's good. Just to see. Probably won't solve a thing, but if we get lucky... we have a whole new area of troubleshooting that opened up. I just want to eliminate everything step by step.

Do you have the emissions crap hooked in? Or did you strip it all out (or leave the connectors hanging)?