1973-1991 Dual fuel tank systems theory of operation

[Popeye75356]

Hatzie,
Thank you for the diagrams and info as I've swapped a 5.3 into my 84' SWB and I've purchased the FiTech Ultimate LS management system. I purchased the 87' baffled tanks along with 87' sending units and Walbro 255 pumps all already installed. My original harness obviously lacks power for the pumps as it ran off of a mechanical. I visited Painless today and one of their engineers drew a diagram for wiring up to the OEM switch as long as the FiTech system supplies the 12v power. My question is simple, is there a vendor that I can source an 87' harness so that I can "plug and play" the wires on the 87' sending units thereby powering my in-tank pumps and sending units when switching tanks or will I be relegated to fabricating my own harness???? Any help would be greatly appreciated. FYI I'm new to all of this ! Father passed away and I felt compelled to attempt a frame off on the only "NEW" truck he ever owned...........

Thanks in advance!
Popeye