2007 5.7 Hemi in 73 Plymouth Duster (pedal cuts out)

I can't help much with this but keep in mind it might be a shielding/ RFI/ EMI issue and or improper grounded/ ground loop. You have completely changed the system (and harness) from OEM, even though (I guess) you are using the OEM system components.
Issue only presents itself in gear, in park I get full pedal.