What you have only has a 0-4V scale from what I have been reading. The SCT XF3 needs an analog range of 0-5V. The other thing that is not defined is the range of AFR from 0-4V so if you can tell me that I might be able to figure it out. On the innovative MTX-L that I have it is 0V=7.35AFR and 5V=22.5 so the equation for this is (v*3.008)+7.35. I"ll check the SCT manual tomorrow for the equation formula.
This is the SCT 1394 cable pin out
This is your Auto Meter 3378 wiring.
Connect to good engine ground.
Brown Wire(Optional Peak/Recall & Warning WOT Switch):
Connect to Wide Open Throttle Switch. Required for
Peak Recall Functionality.
Blue Wire (Optional Data Logger Signal Output):
Connect to signal input(+) or Engine Management System
or Data Acquisition unit.
Blue/Black Wire (Optional Data Logger Signal Ground):
Connect to signal input(-) on Engine Management system
or Data Acquisition Unit.
Connect the Blue Wire (Optional Data Logger Signal Output):
Connect to signal input(+) to the Analog 1 orange wire 3 on the SCT 1394
Connect the Blue/Black Wire (Optional Data Logger Signal Ground):
Connect to signal input(-) to the Analog 1 GND Blue wire 4 on the SCT 1394
I would also tie the grounds to the chassis ground that you have connected the gauge to along with the GND Black wire 2 on the SCT 1394 GND