It does sound like SI 
I did mine the long way by replacing the wires to the starter relay, the starter assist relay and the solenoid with very heavy gauge.
It is easier to mount a new 32a relay down by the starter using the old solenoid wire as it’s trigger and using it to switch power from the un-fused big feed to the starter to the solenoid.
But another contributor to the Ghetto posted about a different fix using a big capacitor to stop voltage drop when the solenoid is fired. I have not got up-to-date comparison on these last two methods