Coil Pack Wiring

I am doing an EFI conversion on a 1975 Fiat 124 Sport with a 1.8L engine. I am wiring the coil pack, which is 4 AMP EFI ING1A coils with integral igniters. I'm using 18 AWG, TXL wire to each coil with Deutsche DTM connector(s). The trigger wires go back to the ECU and require 1 pin each in the connector. The rest of the wires go back to common points, +12V, sensor return (ECU sensor GND), battery ground, and engine head ground. My plan is to tie the common wires together in 4 wire bundles and run a single wire back to each common point. What size wire should I use for this? Can I do this with a DTM connector or do I need to use a DTM for the trigger wires an the sensor return and split the +12V power, battery ground, and head ground out and use a different type connector rated for a higher amperage? Is there any downside to this strategy? The data I can find on the coils leads me to believe that they draw 5 amps each so I think I may need a connector that will handle 20 amps if I use 4 wire groupings. Would I be better off to do the power in two wire groupings with 1 wire out to limit the required amperage rating of the wire and connector to 10 amps? Also for the battery ground should I take that back to one or more of the ECU power ground pins? Should I shield the trigger wires and senor return wire to avoid electrical noise?

Also to clarify a bit, the battery is in the trunk, but my plan is to add a relay/fuse box in the engine compartment for the EFI system. All of the +12V power to the coils, injectors, and sensors will be powered through relays wired from bussed +12V power from the starter battery cable the relay coils will be switched 12V power from the ignition switch. The power will be fused appropriately for the loads to protect the harness. Hopefully that all makes sense.