Lynn again:  Re Light Speed EI EMI:  I talked to Klaus and he sez the Plasma IIIs (the ones you want to use on a turbo 550; the ones I am using) have had no EMI problems when wired correctly.  They can be wired incorrectly by not properly connecting the shielding on the power wires to provide continuity, but when done right the EI can be operated even with storm scopes which are reputed to be very sensitive to EMI.  He recommends about six inches between his control box(s) and any other electronic devices--but here too builders have mounted the box(s) on the radio stack and not had problems.  So I would think an aft firewall position would be no problem for your Chelton, stack or panel--at least I hope not 'cause that is the way mine is built!  Any remaining tech questions, give Klaus a call (best after 9am pacific so he has had time for a couple cups of tea).  don't settle, paul