I would and do use the center pin for the ground but you need 12 V to activate that Ford solenoid, correct? If so then why not use the circuit through the stock Mopar neutral safety switch to provide 12V to the Ford solenoid when it is in park or neutral?
Hopefully some one else on here who has done what I'm talking about will chime in, I'm not sure how much current the stock Mopar neutral switch will handle safely
If your trying to use a 4 pin Ford type starter solenoid maybe one of two small wire posts on them is for the ground from the Ford neutral safety switch
If so your golden using that ground from the stock Mopar neutral switch to provide the ground
IHTHs