Injectors all get 12volts with key on from a wire they all share
Computer has one ground wire for each injector to open it, sequential injection, follows the firing order
Earlier EFI computers did use Batch Fire, just 2 computer grounds each opening 3 injectors(V6) or 4 injectors(V8)
V6 engine wiring would only have 6 injector plugins, V8 computer would have 2 extra
Look at the original injector wires
There would be a red wire on each thats the 12v
If each injector has a different color Ground wire(6 colors) then its a Sequential injection harness
If there are just 2 colors shared then its Batch Fire harness
Mustang V8 wiring here:
https://www.stangnet.com/mustang-forums/attachments/88-91_5-0_eec_wiring_diagram-gif.113534/
It has the 8 different wire colors for injector grounds
You could add those 2 extra wires to computer connector
302 has this firing order, applies to injectors as well as spark plugs, 1 5 4 2 6 3 7 8
1 thru 6 wires should be there, just in the wrong places if still in the V6 harness tape
1, 2 and 3 would be on the passenger side and correct, but 4 would have to be moved to passenger side rear injector
Then 5 and 6 at the front of drivers side
7 could be hooked to 6 and then 8 to 3
But I doubt thats your issue